Efficient proximity labeling in living cells and organisms with TurboID

Application of mass spectrometry for the advancement of PROTACs

The advent of targeted protein degradation technologies, particularly Proteolysis-Targeting Chimeras (PROTACs), enable the selective elimination of target proteins and open up new avenues for the treatment of various diseases. This review delves into the pivotal role of mass spectrometry (MS) in the advancement of PROTACs. MS-based methodologies serve as invaluable tools for identifying PROTAC targets, validating their efficacy, and elucidating ubiquitination sites and protein degradation dynamics. These insights profoundly enrich our comprehension of the mechanisms of action and facilitate the rational design of PROTACs. Furthermore, this review discusses the role of MS in the structural analysis of proteins and the formation of ternary complexes crucial for the activity of PROTACs. The synergy between MS and PROTAC technology holds the promise of groundbreaking advancements in drug discovery by deepening our understanding of the underlying mechanisms that govern PROTAC drug action, thereby promoting the development of innovative strategies for disease treatment.

The proximity proteome of pre-40S pre-ribosomal particle components PNO1 and NOB1 using turboID proximity labeling technology

BACKGROUND

The ribosome assembly factors PNO1 and NOB1 play crucial roles in the maturation of the 40S ribosomal small subunit. TurboID is an efficient biotin ligase that can biotinylate proteins in proximity to the target protein and is widely used to study complex biological processes within cells. In this study, we employed this technology to investigate the complex proximity network of PNO1 and NOB1 within the cell, further revealing their key roles in ribosome biogenesis.

RESULTS

Firstly, through immunofluorescence experiments, we observed that PNO1 and NOB1 have different localizations within the cell. Subsequently, by analyzing the proximal proteins labeled by PNO1-TurboID and NOB1-TurboID, we identified 871 proximal proteins for PNO1 and 1044 for NOB1, with 663 overlapping proteins. Functional analysis revealed that these proximal proteins are predominantly enriched in biological processes related to ribosome assembly, rRNA processing, and translation, all of which are closely linked to ribosome biogenesis. Notably, we validated the mass spectrometry-identified proteins through co-IP experiments and found that PNO1 and NOB1 interact with the translation-related proteins EIF4B and EIF4G2.

CONCLUSION

Our study constructed the protein network environment of ribosome assembly factors PNO1 and NOB1 within the cell and found that their neighboring proteins are primarily involved in key biological processes such as ribosome processing, mRNA translation, and the cell cycle, all of which are critical for ribosome biogenesis. These findings provide a valuable foundation for further understanding the roles of PNO1 and NOB1 in ribosome biogenesis and how they regulate this process.

Heat-shock chaperone HSPB1 mitigates poly-glycine-induced neurodegeneration via restoration of autophagic flux

The CGG repeat expansions in the 5'-UTR regions of certain genes have been implicated in various neurodegenerative and muscular disorders. However, the underlying pathogenic mechanisms are not well understood. In this study, we explore the role of the small molecular chaperone HSPB1 in counteracting neurodegeneration induced by poly-glycine (poly-G) aggregates. Employing a reporter system, we demonstrate that CGG repeat expansions within the 5'-UTR of the GIPC1 gene produce poly-G proteins, by repeat-associated non-AUG (RAN) translation. Through proximity labeling and subsequent mass spectrometry analysis, we characterize the composition of poly-G insoluble aggregates and reveal that these aggregates sequester key macroautophagy/autophagy receptors, SQSTM1/p62 and TOLLIP. This sequestration disrupts MAP1LC3/LC3 recruitment and impairs autophagosome formation, thereby compromising the autophagic pathway. Importantly, we show that HSPB1 facilitates the dissociation of these receptors from poly-G aggregates and consequently restores autophagic function. Overexpressing HSPB1 alleviates poly-G-induced neurodegeneration in mouse models. Taken together, these findings highlight a mechanistic basis for the neuroprotective effects of HSPB1 and suggest its potential as a therapeutic target in treating poly-G-associated neurodegenerative diseases.Abbreviations: AD: Alzheimer disease; AIF1/Iba1: allograft inflammatory factor 1; Baf A1: bafilomycin A1; BFP: blue fluorescent protein; CQ: chloroquine; EIF2A/eIF-2α: eukaryotic translation initiation factor 2A; FRAP: fluorescence recovery after photobleaching; GAPDH: glyceraldehyde-3-phosphate dehydrogenase; GFAP: glial fibrillary acidic protein; GFP: green fluorescent protein; HSPB1: heat shock protein family B (small) member 1; MAP1LC3B/LC3B: microtubule associated protein 1 light chain 3 beta; NOTCH2NLC: notch 2 N-terminal like C; PD: Parkinson disease; PFA: paraformaldehyde; poly-A: poly-alanine; poly-G: poly-glycine; poly-R: poly-arginine; RAN translation: repeat-associated non-AUG translation; RBFOX3/NeuN: RNA binding fox-1 homolog 3; STED: stimulated emission depletion; TARDBP/TDP-43: TAR DNA binding protein; TG: thapsigargin; TOLLIP: toll interacting protein.

Identifying intermolecular interactions in single-molecule localization microscopy

Intermolecular interactions underlie all cellular functions, yet visualizing these interactions at the single-molecule level remains challenging. Single-molecule localization microscopy (SMLM) offers a potential solution. Given a nanoscale map of two putative interaction partners, it should be possible to assign molecules either to the class of coupled pairs or to the class of noncoupled bystanders. Here, we developed a probabilistic algorithm that allows accurate determination of both the absolute number and the proportion of molecules that form coupled pairs. The algorithm calculates interaction probabilities for all possible pairs of localized molecules, selects the most likely interaction set, and corrects for any spurious colocalizations. Benchmarking this approach across a set of simulated molecular localization maps with varying densities (up to ∼55 molecules μm-2) and localization precisions (1 to 50 nm) showed typical errors in the identification of correct pairs of only a few percent. At molecular densities of ∼5 to 10 molecules μm-2 and localization precisions of 20 to 30 nm, which are typical parameters for SMLM imaging, the recall was ∼90%. The algorithm was effective at differentiating between noninteracting and coupled molecules both in simulations and experiments. Finally, it correctly inferred the number of coupled pairs over time in a simulated reaction-diffusion system, enabling determination of the underlying rate constants. The proposed approach promises to enable direct visualization and quantification of intermolecular interactions using SMLM.

Gal-1 promotes lung cancer cell survival by enhancing PARP1/H1.2 interaction to promote DNA repair upon DNA damage response

Galectin-1 (Gal-1), a member of the galectin family, has emerged as a regulator of tumor progression. Several studies have reported the upregulation of Gal-1 expression in multiple cancer cells and its promotion on tumor proliferation. However, the mechanism by which Gal-1 promotes tumor growth remains to be thoroughly understood. In this study, it was discovered that high expression of Gal-1 in various cancers was inversely correlated with the overall survival of patients. Through constructing Gal-1-overexpressing cell lines, it was uncovered that cell proliferation and colony formation were significantly improved. The results of transcriptomic and proximity-labeling-based proteomic analyses indicated that Gal-1 interacted with poly [ADP-ribose] polymerase 1 (PARP1) and histone H1.2 in lung cancer cells. In the case of etoposide treatment leading to DNA double-strand break, Gal-1 accelerated the degradation of H1.2 by enhancing its interaction with PARP1 and promoting its PARylation. It caused the activation of downstream DNA repair pathways such as the serine-protein kinase ATM and nibrin (NBS1) signaling pathways, thus reducing apoptosis, and the Gal-1 inhibitor thiodigalactoside (TDG) could restore cell sensitivity to etoposide. Upon knockdown of Gal-1, DNA damage led to impaired activation of ATM and NBS1 phosphorylation, thereby increasing the sensitivity of the A549 cell line to etoposide. Finally, using a tumor-bearing mouse model, we observed that, in tumors with high Gal-1 expression, the combination treatment of TDG and etoposide significantly inhibited tumor growth. This study provides new clues for the role of Gal-1 in the development of tumors and renders suggestions for the medication of patients with high Gal-1 expression in the clinic.

Identification of the Staphylococcus aureus endothelial cell surface interactome by proximity labeling

Virulence strategies of pathogens depend on interaction with host cells. The binding and activation of receptors located on the plasma membrane are crucial for the attachment to or pathogen internalization by host cells. Identifying host cell receptors is often difficult, and hence, the identity of many proteins that play important roles during host-pathogen interaction remains elusive. We developed a novel proximity labeling approach by decorating the opportunistic pathogen Staphylococcus aureus with ascorbate peroxidase 2. Upon addition of hydrogen peroxide, the peroxidase initiates proximity biotinylation of S. aureus host interaction partners, thereby enabling the identification of these proteins. Here, we demonstrate an endothelial cell surface interactome of 305 proteins, including novel S. aureus co-receptors such as neuronal adhesion molecule, protein tyrosine kinase PTK7, melanotransferrin, protein-tyrosine kinase MET, and CD109. Filtering the interactome for validated surface proteins resulted in a list of 89 protein candidates, 53% of which were described to interact with S. aureus or other pathogens.

IMPORTANCE

Staphylococcus aureus is an opportunistic pathogen that enters host cells such as epithelial or endothelial cells. Intracellular pathogens have been observed in vivo and are thought to serve immune evasion, avoidance of antibiotic treatment, and chronicity of infection. Thus, it is important to understand the mechanisms by which the bacteria are internalized by host cells; however, screening for pathogen-host receptors is difficult. Here, we developed a novel proximity labeling approach, which enabled the identification of several previously unknown host receptors of S. aureus that are engaged during a rapid uptake pathway for the bacteria.

Exploring the complexity of MECP2 function in Rett syndrome

Rett syndrome (RTT) is a neurodevelopmental disorder that is mainly caused by mutations in the methyl-DNA-binding protein MECP2. MECP2 is an important epigenetic regulator that plays a pivotal role in neuronal gene regulation, where it has been reported to function as both a repressor and an activator. Despite extensive efforts in mechanistic studies over the past two decades, a clear consensus on how MECP2 dysfunction impacts molecular mechanisms and contributes to disease progression has not been reached. Here, we review recent insights from epigenomic, transcriptomic and proteomic studies that advance our understanding of MECP2 as an interacting hub for DNA, RNA and transcription factors, orchestrating diverse processes that are crucial for neuronal function. By discussing findings from different model systems, we identify crucial epigenetic details and cofactor interactions, enriching our understanding of the multifaceted roles of MECP2 in transcriptional regulation and chromatin structure. These mechanistic insights offer potential avenues for rational therapeutic design for RTT.

MARK2 regulates Golgi apparatus reorientation by phosphorylation of CAMSAP2 in directional cell migratio

The reorientation of the Golgi apparatus is crucial for cell migration and is regulated by multipolarity signals. A number of non-centrosomal microtubules anchor at the surface of the Golgi apparatus and play a vital role in the Golgi reorientation, but how the Golgi are regulated by polarity signals remains unclear. Calmodulin-regulated spectrin-associated protein 2 (CAMSAP2) is a protein that anchors microtubules to the Golgi, a cellular organelle. Our research indicates that CAMSAP2 is dynamically localized at the Golgi during its reorientation processing. Further research shows that CAMSAP2 is potentially regulated by a polarity signaling molecule called MARK2, which interacts with CAMSAP2. We used mass spectrometry to find that MARK2 phosphorylates CAMSAP2 at serine-835, which affects its interaction with the Golgi-associated protein USO1 but not with CG-NAP or CLASPs. This interaction is critical for anchoring microtubules to the Golgi during cell migration, altering microtubule polarity distribution, and aiding Golgi reorientation. Our study reveals an important signaling pathway in Golgi reorientation during cell migration, which can provide insights for research in cancer cell migration, immune response, and targeted drug development.

p97/VCP is required for piecemeal autophagy of aggresomes

Metazoan cells adapt to the exhaustion of protein quality control (PQC) systems by sequestering aggregation-prone proteins in large, pericentriolar structures termed aggresomes. Defects in both aggresome formation and clearance affect proteostasis and have been linked to neurodegenerative diseases, but aggresome clearance pathways are still underexplored. Here we show that aggresomes comprising endogenous proteins are cleared via selective autophagy requiring the cargo receptor TAX1BP1. TAX1BP1 proximitomes reveal the presence of various PQC systems at aggresomes, including Hsp70 chaperones, the 26S proteasome, and the ubiquitin-selective unfoldase p97/VCP. While Hsp70 and p97/VCP with its cofactors UFD1-NPL4 and FAF1 play key roles in aggresome disassembly, the 26S proteasome is dispensable. We identify aggresomal client proteins that are degraded via different routes, in part in a p97/VCP-dependent manner via aggrephagy. Upon acute inhibition of p97/VCP, aggresomes fail to disintegrate and cannot be incorporated into autophagosomes despite the presence of factors critical for aggrephagosome formation, including p62/SQSTM1, TAX1BP1, and WIPI2. We conclude that the p97/VCP-mediated removal of ubiquitylated aggresomal clients is essential for the disintegration and subsequent piecemeal autophagy of aggresomes.

TurboID Labeling and Analysis of Proteins in the Primary Cilium

Known as the cell's antenna and signaling hub, the primary cilium is a hair-like organelle with a few micrometers in length and 200-300 nm in diameter. Due to the small size of the primary cilium, it is technically challenging to profile ciliary proteins from mammalian cells. Traditional methods, such as physical isolation of cilia, are susceptible to contamination from other cellular components. Other proximity-based labeling methods via APEX or BioID have been used to map ciliary proteins. However, these approaches have their inherent limitations, including the use of toxic reagents like H2O2 and prolonged labeling kinetics. Here, we show a new proximity-based labeling technique for primary cilia with TurboID. TurboID presents a distinct advantage over BioID and APEX2 due to its expedited labeling kinetics, taking minutes instead of hours, and its use of a non-toxic biotin substrate, which eliminates the need for H2O2. When targeted to the cilium, TurboID selectively labels ciliary proteins with biotin. The biotinylated proteins are then enriched with streptavidin beads and labeled with tandem mass tags (TMT), followed by mass spectrometry (MS) detection. This protocol eliminates the requirement of toxic labeling reagents and significantly reduces the labeling time, thus providing advantages in mapping signaling proteins with high temporal resolution in live cells. Key features • Compared to other proximity labeling enzymes, TurboID offers fast labeling kinetics and uses cell-permeable biotin as the labeling reagent [1]. • This protocol includes a straightforward subcellular fractionation step to remove the nuclei to reduce the non-specific background. • This protocol has been successfully applied to the NIH 3T3 cell line and could also be applied in other cell lines and animal tissues.

Proximity labeling and SILAC based proteomic approach identifies proteins at the interface of homotypic and heterotypic cancer cell interactions

Cell-cell interactions are critical for the growth of organisms and maintaining homeostasis. In the tumor microenvironment, these interactions promote cancer progression. Given their importance in healthy and diseased conditions, we have developed a method to analyze the cell-to-cell interactome. Our approach uses enzyme-catalyzed proximity labeling and SILAC-based proteomics to identify the proteins involved in cancer cell interactions. By targeting HRP to the outer leaflet of the plasma membrane in bait cells, we were able to label the neighboring prey cells and distinguish between the proteomes of bait and prey cells using SILAC labeling in a co-culture system. We mapped both the homotypic and heterotypic interactomes of epithelial and mesenchymal breast cancer cells. The enrichment of cell surface and extracellular proteins confirms the specificity of our methodology. We further verified selected hits from different cell-cell interactomes in co-cultures using microscopy. This method revealed prominent signaling pathways orchestrating homotypic and heterotypic interactions of epithelial and mesenchymal cells. It also highlights the importance of exosomes in these interactions. Our methodology can be applied to any type of cell-cell interaction in 2D co-culture or 3D tumor models.

Aberrant phase separation drives membranous organelle remodeling and tumorigenesis

Membrane remodeling is essential for numerous cellular functions. Although liquid-liquid phase separation (LLPS) of intrinsically disordered region (IDR)-rich proteins could drive dramatic membrane remodeling of artificial giant unilamellar vesicles, it remains elusive whether LLPS-mediated membrane-remodeling functions in live cells and what role it plays in specific bioprocesses. Here, we show that three IDR-rich integral transmembrane fusion proteins (MFPs), generated by chromosomal translocations, can lead to de novo remodeling of their located membranous organelles. Taking FUS-CREB3L2, prevalent in low-grade fibromyxoid sarcoma (LGFMS), as a proof of concept, we recorded super-resolution long-time imaging of endoplasmic reticulum (ER) remodeling dynamics as accumulating FUS-CREB3L2, meanwhile causing spontaneous ER stress to hijack the X-box-binding protein 1 (XBP1) pathway. We further reveal the underlying mechanisms of how FUS-CREB3L2 transduces its tumorigenic signals and aberrant LLPS effects from the ER membrane into the nucleus autonomously, which activates hundreds of LGFMS-specific genes de novo compared with CREB3L2, thus sufficiently reprogramming the cells into an LGFMS-like status.

Identification of multiple novel procoagulant plasma ligands for stabilin-2

BACKGROUND

Damaging STAB2 gene variants are associated with increased venous thromboembolic risk. STAB2 encodes stabilin-2, a clearance receptor, expressed by the liver and spleen. Given its function, it is likely that the prothrombotic state associated with stabilin-2 deficiency is due to reduced procoagulant protein clearance, but the identity of these ligands is unknown.

OBJECTIVES

To identify plasma stabilin-2 ligands using proximity biotinylation proteomics.

METHODS

Cells stably expressing stabilin-2-TurboID were incubated with human plasma and biotin to initiate TurboID labeling of plasma ligands in endocytic vesicles. Biotinylated proteins were purified and identified using mass spectrometry. Candidate plasma ligands with roles in hemostasis were fluorescently labeled and incubated with stabilin-2 expressing and control cells. Flow cytometry assessed ligand surface binding and confocal microcopy assessed colocalization with stabilin-2 and lysosomes. Furthermore, plasma levels of ligands were measured in Stab2-deficient mice and littermate controls.

RESULTS

Twenty-eight stabilin-2 specific ligands were identified. Interactions with von Willebrand factor, fibrinogen, pro(thrombin), heparin cofactor II, high molecular weight kininogen, plasminogen, and C4b-binding protein were probed. Heparin cofactor II, high molecular weight kininogen, plasminogen, and fibrinogen showed binding to stabilin-2 using flow cytometry (>2-fold higher than controls). Confocal microscopy demonstrated stabilin-2 dependent colocalization of all ligands with lysosomes. In Stab2-deficient mice, ligand levels were not significantly increased, suggesting in mice stabilin-2 is not their main clearance receptor.

CONCLUSION

These results confirm the value of proximity labeling proteomics in identifying receptor ligands and suggest damaging STAB2 variants may increase venous thromboembolic risk potentially through altered hemostatic protein clearance.

Locus-specific chromatin proteomics using dCas-guided proximity labelling in Aspergillus nidulans

Proximity labelling that uses promiscuous biotin ligases (BirA) fused to a bait protein is a powerful tool to identify protein interaction partners in vivo under different metabolic or developmental conditions. BirA can also be used to determine protein composition and interaction partners at specific chromatin locations when it is fused with enzymatically-disabled Cas9 (dCas9) and then guided to the location of interest by sgRNAs. We adapted this method (called CasID) for fungal cells using the nitrate assimilation gene cluster of A. nidulans as a model locus and estrogen-inducible expression of the dCas9-BirA fusion to improve condition-specific labelling. For method establishment, we first verified the presence of dCas-BirA and a known transcription factor at the nitrate locus by chromatin immunoprecipitation (ChIP). Results show that both dCas-BirA and the AreA transcription factor are present at the locus of interest under the conditions used for biotinylation. We then optimized the CasID procedure for efficient labelling and background reduction using the CasID-sgRNA strain and two control strains, one lacking the sgRNA and another one lacking the whole CasID system. Here we provide proof-of-concept for the suitability of the method by showing that biotinylated proteins are enriched in the CasID strains in comparison to the controls. After background reduction, 32 proteins remained in two independent experiments exclusively enriched in the Cas-ID-sgRNA strain. Among these proteins was NmrA, an AreA-interacting regulator, and we also found several chromatin-associated proteins. Overall, our results demonstrate that CasID is suitable for locus-specific labelling and identification of chromatin-associated proteins and transcription factors in A. nidulans. However, the high background of proteins that are biotinylated out of chromatin context or unspecifically attach to the affinity purification matrix needs to be addressed by implementing a set of rigorous controls. In summary, we herewith provide a detailed protocol for application of the method that proved to be useful for the identification of novel chromatin-associated proteins and their interaction partners at a specific genomic locus in divers metabolic and developmental conditions. AUTHOR SUMMARY: This study demonstrates that locus-specific proteomics can be carried out by dCas-BirA guided proximity labelling in Aspergillus nidulans. For establishment, we targeted the well-described bidirectional promoter region between niaD, a nitrate reductase, and niiA, a nitrite reductase. At this locus we could test by chromatin immunoprecipitation (ChIP) in combination with qPCR if both, the dCas9-BirA fusion as well as a central transcription factor are at the locus under the conditions of our CasID experiment. After this first control step, we considered that unspecific labelling by dCas-BirA during the time from translation to landing at the targeted chromatin locus may be one of the most relevant drawbacks of the method. Therefore, we developed a number of control strains that would allow us to clearly discriminate between background and sgRNA-dependent specific labelling at the locus. Our protein MS results validated these estimates and only considering the results of these controls enabled us to distinguish the set of locus-specific proteins from a very high general background. Finally, enrichment of biotinylated proteins through affinity purification with streptavidin resin and subsequent LC-MS/MS analysis showed that more than 800 proteins were detected in each sample, emphasizing the high background of the purification method. After background reduction of the control samples, we were able to identify 32 proteins which were exclusively detected in the test strain in two independent measurements, including several chromatin-associated proteins and NmrA, a negative regulator of the nitrate locus transcription factor AreA.

A dual-purification system to isolate mitochondrial subpopulations

Mitochondria perform diverse functions, including producing ATP through oxidative phosphorylation, synthesizing macromolecule precursors, maintaining redox balance among many others. Given this diversity of functions, we and others have hypothesized that cells maintain specialized subpopulations of mitochondria. To begin addressing this hypothesis, we developed a new dual-purification system to isolate subpopulations of mitochondria for chemical and biochemical analyses. We used APEX2 proximity labeling such that mitochondria were biotinylated based on proximity to another organelle. All mitochondria were isolated by an elutable MitoTag-based affinity precipitation system. Biotinylated mitochondria were then purified using immobilized avidin. We used this system to compare the proteomes of endosome- and lipid droplet-associated mitochondria in U-2 OS cells, which demonstrated that these subpopulations were indistinguishable from one another but were distinct from the global mitochondria proteome. Our results suggest that this purification system could aid in describing subpopulations that contribute to intracellular mitochondrial heterogeneity, and that this heterogeneity might be more substantial than previously imagined.

Ligand-induced ubiquitination unleashes LAG3 immune checkpoint function by hindering membrane sequestration of signaling motifs

Lymphocyte activation gene 3 (LAG3) has emerged as a promising cancer immunotherapy target, but the mechanism underlying LAG3 activation upon ligand engagement remains elusive. Here, LAG3 was found to undergo robust non-K48-linked polyubiquitination upon ligand engagement, which promotes LAG3's inhibitory function instead of causing degradation. This ubiquitination could be triggered by the engagement of major histocompatibility complex class II (MHC class II) and membrane-bound (but not soluble) fibrinogen-like protein 1 (FGL1). LAG3 ubiquitination, mediated redundantly by the E3 ligases c-Cbl and Cbl-b, disrupted the membrane binding of the juxtamembrane basic residue-rich sequence, thereby stabilizing the LAG3 cytoplasmic tail in a membrane-dissociated conformation enabling signaling. Furthermore, LAG3 ubiquitination is crucial for the LAG3-mediated suppression of antitumor immunity in vivo. Consistently, LAG3 therapeutic antibodies repress LAG3 ubiquitination, correlating with their checkpoint blockade effects. Moreover, patient cohort analyses suggest that LAG3/CBL coexpression could serve as a biomarker for response to LAG3 blockade. Collectively, our study reveals an immune-checkpoint-triggering mechanism with translational potential in cancer immunotherapy.

Delineating cysteine-reactive compound modulation of cellular proteostasis processes

Covalent modulators and covalent degrader molecules have emerged as drug modalities with tremendous therapeutic potential. Toward realizing this potential, mass spectrometry-based chemoproteomic screens have generated proteome-wide maps of potential druggable cysteine residues. However, beyond these direct cysteine-target maps, the full scope of direct and indirect activities of these molecules on cellular processes and how such activities contribute to reported modes of action, such as degrader activity, remains to be fully understood. Using chemoproteomics, we identified a cysteine-reactive small molecule degrader of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) nonstructural protein 14 (nsp14), which effects degradation through direct modification of cysteines in both nsp14 and in host protein disulfide isomerases. This degrader activity was further potentiated by generalized electrophile-induced global protein ubiquitylation, proteasome activation and widespread aggregation and depletion of host proteins, including the formation of stress granules. Collectively, we delineate the wide-ranging impacts of cysteine-reactive electrophilic compounds on cellular proteostasis processes.

Uncovering the electrical synapse proteome in retinal neurons via in vivo proximity labeling

Electrical synapses containing Connexin 36 (Cx36) represent the main means for direct electrical communication among neurons in the mammalian nervous system. However, little is known about the protein complexes that constitute these synapses. In the present study, we applied different BioID strategies to screen the interactomes of Connexin 36 and its zebrafish orthologue Cx35b in retinal neurons. For in vivo proximity labeling in mice, we took advantage of the Cx36-EGFP strain and expressed a GFP-nanobody-TurboID fusion construct selectively in AII amacrine cells. For in vivo BioID in zebrafish, we generated a transgenic line expressing a Cx35b-TurboID fusion under control of the Cx35b promoter. Both strategies allowed us to capture a plethora of molecules that were associated with electrical synapses and showed a high degree of evolutionary conservation in the proteomes of both species. Besides known interactors of Cx36 such as ZO-1 and ZO-2 we have identified more than 50 new proteins, such as scaffold proteins, adhesion molecules and regulators of the cytoskeleton. Moreover, we determined the subcellular localization of these proteins in mouse retina and tested potential binding interactions with Cx36. Amongst these new interactors, we identified signal induced proliferation associated 1 like 3 (Sipa1l3), a protein that has been implicated in cell junction formation and cell polarity, as a new scaffold of electrical synapses. Interestingly, Sipa1l3 was able to interact with ZO-1, ZO-2 and Cx36, suggesting a pivotal role in electrical synapse function. In summary, our study provides the first detailed view of the electrical synapse proteome in retinal neurons, which is likely to apply to electrical synapses elsewhere.

Protein Engineering for Spatiotemporally Resolved Cellular Monitoring

Protein engineering has been extensively applied to the development of genetically encoded reporters for spatiotemporally resolved monitoring of dynamic biochemical activity across cellular compartments in living cells. Genetically encoded reporters facilitate the visualization and recording of cellular processes, including transmission of signaling molecules, protease activity, and protein-protein interactions. In this review, we describe and assess common reporter motifs and protein engineering strategies for designing genetically encoded reporters. We also discuss essential parameters for evaluating genetically encoded reporters, along with future protein engineering opportunities in this field.

Mapping the Interactome of OSCC Prognostic-Associated Proteins NDRG1 and PGK1 Through Proximity Labeling Using TurboID

Oral squamous cell carcinoma (OSCC) is a prevalent type of head and neck cancer, comprising over 90% of all oral malignancies worldwide. The identification of diagnostic and prognostic markers for OSCC is crucial for improving patient outcomes, as early detection and treatment are critical for the successful management of this disease. Previously, we demonstrated that N-myc downstream-regulated gene 1 (NDRG1) and phosphoglycerate kinase 1 (PGK1) are prognostic markers for OSCC; however, their role in OSCC development remains unclear. To investigate this, we used TurboID-based proximity labeling to identify the interactomes of NDRG1 and PGK1 in HEK293 cells. Herein, protein abundance patterns from three time points were used for clustering 364 proteins with a "fast" or "slow" response to biotin. Of these, 65 proteins were also identified in neoplastic islands of OSCC patients from our previous study, and 28 of these proteins have their gene expression associated with prognostic features, including death, metastasis, and relapse. PRM-MS enabled the quantification of 17 of these proteins, providing further evidence of their presence in the OSCC prognostic interactome. Finally, we characterized a prognostic-associated interactome composed of 28 proteins, which enabled the prioritization of candidates that can be further explored in OSCC progression. The mass spectrometry data generated in this study have been deposited in ProteomeXchange with the data set identifier PXD048046.

Molecular organization of central cholinergic synapses

Synapses have undergone significant diversification and adaptation, contributing to the complexity of the central nervous system. Understanding their molecular architecture is essential for deciphering the brain's functional evolution. While nicotinic acetylcholine receptors (nAchRs) are widely distributed across metazoan brains, their associated protein networks remain poorly characterized. Using in vivo proximity labeling, we generated proteomic maps of subunit-specific nAchR interactomes in developing and mature Drosophila brains. Our findings reveal a developmental expansion and reconfiguration of the nAchR interactome. Proteome profiling with genetic perturbations showed that removing individual nAchR subunits consistently triggers compensatory shifts in receptor subtypes, highlighting mechanisms of synaptic plasticity. We also identified the Rho-GTPase regulator Still life (Sif) as a key organizer of cholinergic synapses, with loss of Sif disrupting their molecular composition and structural integrity. These results provide molecular insights into the development and plasticity of central cholinergic synapses, advancing our understanding of synaptic identity conservation and divergence.

TAF2 condensation in nuclear speckles links basal transcription factor TFIID to RNA splicing factors

TFIID is an essential basal transcription factor, crucial for RNA polymerase II (pol II) promoter recognition and transcription initiation. The TFIID complex consists of the TATA binding protein (TBP) and 13 TBP-associated factors (TAFs) that contain intrinsically disordered regions (IDRs) with currently unknown functions. Here, we show that a conserved IDR drives TAF2 to nuclear speckle condensates independently of other TFIID subunits. Quantitative mass spectrometry analyses reveal TAF2 proximity to RNA splicing factors including specific interactions of the TAF2 IDR with SRRM2 in nuclear speckles. Deleting the IDR from TAF2 does not majorly impact global gene expression but results in changes of alternative splicing events. Further, genome-wide binding analyses suggest that the TAF2 IDR impedes TAF2 promoter association by guiding TAF2 to nuclear speckles. This study demonstrates that an IDR within the large multiprotein complex TFIID controls nuclear compartmentalization and thus links distinct molecular processes, namely transcription initiation and RNA splicing.

Identification of MEF2A, MEF2C, and MEF2D interactomes in basal and Fsk-stimulated mouse MA-10 Leydig cells

BACKGROUND

Myocyte enhancer factor 2 transcription factors regulate essential transcriptional programs in various cell types. The activity of myocyte enhancer factor 2 factors is modulated through interactions with cofactors, chromatin remodelers, and other regulatory proteins, which are dependent on cell context and physiological state. In steroidogenic Leydig cells, MEF2A, MEF2C, and MEF2D are key regulators of genes involved in steroid hormone synthesis, reproductive function, and oxidative stress defense. However, the specific network of myocyte enhancer factor 2-interacting proteins in Leydig cells remains unknown.

OBJECTIVE

To identify the interactome of each MEF2 factor present in Leydig cells.

MATERIALS AND METHODS

TurboID proximity-mediated biotinylation combined with mass spectrometry and bioinformatic analyses were used to identify the protein‒protein interaction networks of MEF2A, MEF2C, and MEF2D in MA-10 Leydig cells under basal and stimulated conditions.

RESULTS

We identified 109 potential myocyte enhancer factor 2-interacting proteins, including some previously known myocyte enhancer factor 2 partners. The interactome for each myocyte enhancer factor 2 factor is dynamic and exhibits unique and shared interaction networks between basal and stimulated conditions. Further analysis through Gene Ontology and Kyoto Encyclopedia of Genes and Genomes pathway enrichment categorized these interactions, revealing involvement in pathways related to cellular metabolism, transcriptional regulation, and steroidogenesis.

DISCUSSION AND CONCLUSION

These findings suggest that myocyte enhancer factor 2 factors can participate in diverse transcriptional activities, capable of gene activation or repression, depending on different protein‒protein interactions. In addition, the differential interactome for each myocyte enhancer factor 2 factor suggests unique regulatory roles for each factor in modulating Leydig cell function. Overall, this study provides new mechanistic insights into myocyte enhancer factor 2 action in Leydig cells by identifying interacting partners that likely influence their functions.

TRIM52 maintains cellular fitness and is under tight proteolytic control by multiple giant E3 ligases

Tripartite motif 52 (TRIM52) exhibits strong positive selection in humans, yet is lost in many other mammals. In contrast to what one would expect for such a non-conserved factor, TRIM52 loss compromises cell fitness. We set out to determine the cellular function of TRIM52. Genetic and proteomic analyses revealed TRIM52 physically and functionally interacts with the DNA repair machinery. Our data suggest that TRIM52 limits topoisomerase 2 adducts, thereby preventing cell-cycle arrest. Consistent with a fitness-promoting function, TRIM52 is upregulated in various cancers, prompting us to investigate its regulatory pathways. We found TRIM52 to be targeted for ultra-rapid proteasomal degradation by the giant E3 ubiquitin ligases BIRC6, HUWE1, and UBR4/KCMF1. BIRC6 mono-ubiquitinates TRIM52, with subsequent extension by UBR4/KCMF1. These findings suggest a role for TRIM52 in maintaining genome integrity, and regulation of its own abundance through multi-ligase degradation.

Engineered Proteins and Chemical Tools to Probe the Cell Surface Proteome

The cell surface proteome, or surfaceome, is the hub for cells to interact and communicate with the outside world. Many disease-associated changes are hard-wired within the surfaceome, yet approved drugs target less than 50 cell surface proteins. In the past decade, the proteomics community has made significant strides in developing new technologies tailored for studying the surfaceome in all its complexity. In this review, we first dive into the unique characteristics and functions of the surfaceome, emphasizing the necessity for specialized labeling, enrichment, and proteomic approaches. An overview of surfaceomics methods is provided, detailing techniques to measure changes in protein expression and how this leads to novel target discovery. Next, we highlight advances in proximity labeling proteomics (PLP), showcasing how various enzymatic and photoaffinity proximity labeling techniques can map protein-protein interactions and membrane protein complexes on the cell surface. We then review the role of extracellular post-translational modifications, focusing on cell surface glycosylation, proteolytic remodeling, and the secretome. Finally, we discuss methods for identifying tumor-specific peptide MHC complexes and how they have shaped therapeutic development. This emerging field of neo-protein epitopes is constantly evolving, where targets are identified at the proteome level and encompass defined disease-associated PTMs, complexes, and dysregulated cellular and tissue locations. Given the functional importance of the surfaceome for biology and therapy, we view surfaceomics as a critical piece of this quest for neo-epitope target discovery.

Spatial proteomics of ER tubules reveals CLMN, an ER-actin tether at focal adhesions that promotes cell migration

The endoplasmic reticulum (ER) is structurally and functionally diverse, yet how its functions are organized within morphological subdomains is incompletely understood. Utilizing TurboID-based proximity labeling and CRISPR knockin technologies, we map the proteomic landscape of the human ER network. Sub-organelle proteomics reveals enrichments of proteins into ER tubules, sheets, and the nuclear envelope. We uncover an ER-enriched actin-binding protein, calmin/CLMN, and define it as an ER-actin tether that localizes to focal adhesions adjacent to ER tubules. Mechanistically, we find that CLMN depletion perturbs adhesion disassembly, actin dynamics, and cell movement. CLMN-depleted cells display decreased polarization of ER-plasma membrane contacts and calcium signaling factor STIM1 and altered calcium signaling near ER-actin interfaces, suggesting that CLMN influences calcium signaling to facilitate F-actin/adhesion dynamics. Collectively, we map the sub-organelle proteome landscape of the ER, identify CLMN as an ER-actin tether, and describe a non-canonical mechanism by which ER tubules engage actin to regulate cell migration.

SWI/SNF complex-mediated ZNF410 cooperative binding maintains chromatin accessibility and enhancer activity

The clustering of multiple transcription factor binding sites (TFBSs) for the same TF has proved to be a pervasive feature of cis-regulatory elements in the eukaryotic genome. However, the contribution of binding sites within the homotypic clusters of TFBSs (HCTs) to TF binding and target gene expression remains to be understood. Here, we characterize the CHD4 enhancers that harbor unique functional ZNF410 HCTs genome wide. We uncover that ZNF410 controls chromatin accessibility and activity of the CHD4 enhancer regions. We demonstrate that ZNF410 binds to the HCTs in a collaborative fashion, further conferring transcriptional activation. In particular, three ZNF410 motifs (sub-HCTs) located at 3' end of the distal enhancer act as "switch motifs" to control chromatin accessibility and enhancer activity. Mechanistically, the SWI/SNF complex is selectively required to mediate cooperative ZNF410 binding for CHD4 expression. Together, our findings expose a complex functional hierarchy of homotypic clustered motifs, which cooperate to fine-tune target gene expression.

Compartmentalized localization of perinuclear proteins within germ granules in C. elegans

Germ granules, or nuage, are RNA-rich condensates that are often docked on the cytoplasmic surface of germline nuclei. C. elegans perinuclear germ granules are composed of multiple subcompartments, including P granules, Mutator foci, Z granules, SIMR foci, P -bodies, and E granules. Although many perinuclear proteins have been identified, their precise localization within the subcompartments of the germ granule is still unclear. Here, we systematically labeled perinuclear proteins with fluorescent tags via CRISPR-Cas9 technology. Using this nematode strain library, we identified a series of proteins localized in Z or E granules and extended the characterization of the D granule. Finally, we found that the LOTUS domain protein MIP-1/EGGD-1 regulated the multiphase organization of the germ granule. Overall, our work identified the germ-granule architecture and redefined the compartmental localization of perinuclear proteins. Additionally, the library of genetically modified nematode strains will facilitate research on C. elegans germ granules.

Mapping RNA-Protein Interactions via Proximity Labeling-Based Approaches

RNA-protein interactions are fundamental to a wide range of biological processes, and understanding these interactions in their native cellular context is both vital and challenging. Traditional methods for studying RNA-protein interactions rely on crosslinking, which can introduce artifacts. Recently, proximity labeling-based techniques have emerged as powerful alternatives, offering a crosslinking-free approach to investigate these interactions. This review highlights recent advancements in the development and application of proximity labeling methods, focusing on both RNA-centric and protein-centric strategies for profiling cellular RNA-protein interactions. By examining these innovative approaches, we aim to provide insights into their potential for enhancing our understanding of RNA-protein dynamics in various biological settings.

Mechanical control of the alternative splicing factor PTBP1 regulates extracellular matrix stiffness induced proliferation and cell spreading

Cells sense mechanical cues and convert them into biochemical responses to regulate biological processes such as embryonic development, aging, cellular homeostasis, and disease progression. In this study, we introduce a large-scale, systematic approach to identify proteins with mechanosensitive nuclear localization, highlighting their potential roles in mechanotransduction. Among the proteins identified, we focus here on the splicing factor PTBP1. We demonstrate that its nuclear abundance is regulated by mechanical cues such as cell density, size, and extracellular matrix (ECM) stiffness and that PTBP1 medicates the mechanosensitive alternative splicing of the endocytic adapter protein Numb. Furthermore, we show that PTBP1 and Numb alternative splicing is critical for ECM stiffness-induced epithelial cell spreading and proliferation as well as for mesenchymal stem cell differentiation into osteoblasts on a stiff matrix. Our results underscore the emerging role of alternative splicing in mechanotransduction and provide novel mechanistic insights into how matrix stiffness modulates cellular mechanoresponses.

Unveiling the substrate specificity of the ABC transporter Tba and its role in glycopeptide biosynthesis

Glycopeptide antibiotics (GPA) such as vancomycin are essential last-resort antibiotics produced by actinomycetes. Their biosynthesis is encoded within biosynthetic gene clusters, also harboring genes for regulation, and transport. Diverse types of GPAs have been characterized that differ in peptide backbone composition and modification patterns. However, little is known about the ATP-binding cassette (ABC) transporters facilitating GPA export. Employing a multifaceted approach, we investigated the substrate specificity of GPA ABC-transporters toward the type-I GPA balhimycin. Phylogenetic analysis suggested and trans-complementation experiments confirmed that balhimycin is exported only by the related type I GPA transporters Tba and Tva (transporter of vancomycin). Molecular dynamics simulations and mutagenesis experiments showed that Tba exhibits specificity toward the peptide backbone rather than the modifications. Unexpectedly, deletion or functional inactivation of Tba halted balhimycin biosynthesis. Combined with proximity biotinylation experiments, this suggested that the interaction of the active transporter with the biosynthetic machinery is required for biosynthesis.

Selective identification of epigenetic regulators at methylated genomic sites by SelectID

DNA methylation is a significant component in proximal chromatin regulation and plays crucial roles in regulating gene expression and maintaining the repressive state of retrotransposon elements. However, accurate profiling of the proteomics which simultaneously identifies specific DNA sequences and their associated epigenetic modifications remains a challenge. Here, we report a strategy termed SelectID (selective profiling of epigenetic control at genome targets identified by dCas9), which introduces methylated DNA binding domain into dCas9-mediated proximity labeling system to enable in situ protein capture at repetitive elements with 5-methylcytosine (5mC) modifications. SelectID is demonstrated as feasible as dCas9-TurboID system at specific DNA methylation regions, such as the chromosome 9 satellite. Using SelectID, we successfully identify CHD4 as potential repressors of methylated long interspersed nuclear element-1 (LINE-1) retrotransposon through direct binding at the 5' untranslated region (5'UTR) of young LINE-1 elements. Overall, our SelectID approach has opened up avenues for uncovering potential regulators of specific DNA regions with DNA methylation, which will greatly facilitate future studies on epigenetic regulation.

Mitochondrial complexity is regulated at ER-mitochondria contact sites via PDZD8-FKBP8 tethering

Mitochondria-ER membrane contact sites (MERCS) represent a fundamental ultrastructural feature underlying unique biochemistry and physiology in eukaryotic cells. The ER protein PDZD8 is required for the formation of MERCS in many cell types, however, its tethering partner on the outer mitochondrial membrane (OMM) is currently unknown. Here we identify the OMM protein FKBP8 as the tethering partner of PDZD8 using a combination of unbiased proximity proteomics, CRISPR-Cas9 endogenous protein tagging, Cryo-electron tomography, and correlative light-electron microscopy. Single molecule tracking reveals highly dynamic diffusion properties of PDZD8 along the ER membrane with significant pauses and captures at MERCS. Overexpression of FKBP8 is sufficient to narrow the ER-OMM distance, whereas independent versus combined deletions of these two proteins demonstrate their interdependence for MERCS formation. Furthermore, PDZD8 enhances mitochondrial complexity in a FKBP8-dependent manner. Our results identify a novel ER-mitochondria tethering complex that regulates mitochondrial morphology in mammalian cells.

Usutu virus NS4A induces autophagy and is targeted by the selective autophagy receptor p62/SQSTM1 for degradation

Usutu virus (USUV) is an emerging orthoflavivirus, which mainly affects birds but in rare cases can cause severe neuroinvasive disease in humans. The virus relies on a multitude of host cell proteins, molecules and cellular processes for its replication, and must subvert host antiviral responses to establish a successful infection. Studying the complex network of virus-host protein interactions by proteomics approaches can therefore provide new insights in the replication cycle of USUV and its pathogenesis. We have previously shown that the USUV protein NS4A acts as an antagonist of the antiviral interferon response, and here we further map the host interaction partners of USUV NS4A using proximity labeling coupled to mass spectrometry. The resulting NS4A interactome revealed many host proteins involved in the autophagy pathway. We showed that both USUV infection and overexpression of USUV NS4A can indeed induce the autophagy pathway. However, stimulation or inhibition of the autophagy pathway in general did not affect USUV replication. Therefore, we decided to specifically analyze the role of the selective autophagy receptor sequestosome 1 (p62/SQSTM1), since we identified this protein as an important interaction partner of USUV NS4A. We found that p62 is involved in the degradation of USUV NS4A. In agreement with this, the knockdown of p62 enhanced replication of USUV in A549 cells. P62 thus plays an antiviral role during USUV infection, although this antiviral effect might also be related to its functions outside the autophagy pathway, such as modulation of the immune response. In conclusion, this study showed that USUV NS4A induces autophagy and is then targeted by p62 for degradation by the autophagic machinery, uncovering a new role of p62 in the antiviral defense against USUV.

Characterizing host-microbe interactions with bacterial effector proteins using proximity-dependent biotin identification (BioID)

Bacterial pathogens have evolved diverse strategies to manipulate host cells to establish infection. At a molecular level, this is often mediated by virulence factors that are secreted into host cells (herein referred to as effectors), which target host cellular pathways by initiating host-pathogen protein-protein interactions that alter cellular function in the host. By establishing this network of host-pathogen protein-protein interactions, pathogenic bacteria modulate and hijack host cell processes for the benefit of the pathogen, ultimately promoting survival, replication, and cell-to-cell spread within the host. Effector proteins also mediate diverse host-microbe interactions in nature, contributing to symbiotic relationships spanning from mutualism to commensalism to parasitism. While effector proteins play crucial roles in nature, molecular properties such as the transient nature of the underlying protein-protein interactions and their affinity for targeting host biological membranes often presents challenges to elucidating host targets and mechanism of action. Proximity-dependent biotin identification (termed BioID) has proven to be a valuable tool in the field of cell biology to identify candidate protein-protein interactions in eukaryotic cells, yet has remained relatively underexploited by bacterial pathogenesis researchers. Here, we discuss bacterial effector function at a molecular level, and challenges presented by traditional approaches to host target identification. We highlight the BioID approach and its potential strengths in the context of identifying host-pathogen protein-protein interactions, and explore BioID's implementation to study host-microbe interactions mediated by bacteria. Collectively, BioID represents a powerful tool for the study of bacterial effector proteins, providing new insight into our understanding of pathogenesis and other symbiotic relationships, and opportunities to identify new factors that contribute to host response to infection.

Site-2 protease Sll0528 interacts with RbcR to regulate carbon/nitrogen homeostasis in the cyanobacterium Synechocystis sp. PCC 6803

Cyanobacteria play pivotal roles in global biogeochemical cycles through oxygenic photosynthesis. To maintain cellular homeostasis, these organisms utilize sophisticated acclimation mechanisms to adapt to environmental fluctuations, particularly concerning nitrogen availability. While nitrogen deprivation induces dormancy, excess ammonium can have toxic effects on cyanobacteria and other photosynthetic organisms-a phenomenon for which the acclimation mechanisms remain poorly understood. Through the physiological characterization of knockout and overexpression mutants in Synechocystis sp. PCC 6803, we identified the site-2 protease Sll0528 as a critical regulator of ammonium stress acclimation. TurboID-based proximity labeling, coupled with quantitative proteomics, revealed a robust set of putative Sll0528-interacting proteins, some of which were subsequently validated through bacterial two-hybrid assays and transcriptomic profiling. Notably, we confirmed the physical interaction between Sll0528 and RbcR, a low-carbon-responsive transcriptional regulator. Transcriptomic analysis showed that the knockout of sll0528 led to a significant downregulation of the RbcR regulon, including the ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) operon rbcLXS. Further analysis suggests that this downregulation might result from improper post-transcriptional regulation of RbcR, which depends on its interaction with Sll0528. Our findings reveal novel regulatory crosstalk between a cyanobacterial S2P protease and the carbon-responsive transcriptional machinery, providing new mechanistic insights into the control of cyanobacterial carbon-nitrogen homeostasis during nitrogen fluctuations. This study offers insights into the functional characterization of other S2P proteases in photosynthetic organisms and may facilitate the cyanobacteria-based bioremediation of ammonium-rich wastewater.

The metabolic enzyme GYS1 condenses with NONO/p54nrb in the nucleus and spatiotemporally regulates glycogenesis and myogenic differentiation

Accumulating evidence indicates that metabolic enzymes can directly couple metabolic signals to transcriptional adaptation and cell differentiation. Glycogen synthase 1 (GYS1), the key metabolic enzyme for glycogenesis, is a nucleocytoplasmic shuttling protein compartmentalized in the cytosol and nucleus. However, the spatiotemporal regulation and biological function of nuclear GYS1 (nGYS1) microcompartments remain unclear. Here, we show that GYS1 dynamically reorganizes into nuclear condensates under conditions of glycogen depletion or transcription inhibition. nGYS1 complexes with the transcription factor NONO/p54nrb and undergoes liquid-liquid phase separation to form biomolecular condensates, leading to its nuclear retention and inhibition of glycogen biosynthesis. Compared to their wild-type littermates, Nono-deficient mice exhibit exercise intolerance, higher muscle glycogen content, and smaller myofibers. Additionally, Gys1 or Nono deficiency prevents C2C12 differentiation and cardiotoxin-induced muscle regeneration in mice. Mechanistically, nGYS1 and NONO co-condense with the myogenic transcription factor MyoD and preinitiation complex (PIC) proteins to form transcriptional condensates, driving myogenic gene expression during myoblast differentiation. These results reveal the spatiotemporal regulation and subcellular function of nuclear GYS1 condensates in glycogenesis and myogenesis, providing mechanistic insights into glycogenoses and muscular dystrophy.

AgrC biotinylation inhibits Staphylococcus aureus infection

Staphylococcus aureus (S. aureus) is a leading cause of nosocomial infections, particularly among antibiotic-resistant strains. S. aureus virulence is governed by the accessory gene regulator (Agr) quorum sensing (QS) system, which relies on AgrC, a two-component histidine kinase, to detect secreted auto-inducing peptides (AIPs). Emerging evidence highlights the potential of inhibiting the interaction between AgrC and AIPs as a promising therapeutic strategy. Given the limited clinic methods in inhibiting AgrC, we hereby report a novel method utilizing TurboID, an engineered biotin ligase, to inhibit Agr C on S. aureus via its biotinylation. To achieve this goal, a fusion protein named TurboID-AgrD[Formula: see text] (Agr-ID) was designed to include an AgrC binding domain (AgrID[Formula: see text]) and a catalytic domain (TurboID) for AgrC biotinylation. By incubating with Alexa Fluor 647-conjugated streptavidin, the biotinylated AgrC on S. aureus was successfully visualized through fluorescence microscopy with 100x objective. We further confirmed the specific biotinylation of AgrC using Western Blotting, and biotinylated AgrC resulted in inhibiting the growth of S. aureus strains, including S. aureus 25923, S. aureus 43300, and S. aureus 6538 (MRSA). The downstream biological effect of AgrC biotinylation exhibited decreased virulence protein generation as monitored by the lower presence of apoptotic HEK 293T cells after incubating with S. aureus cell lysates and supernatant. The impaired colonizing features from biotinylated S. aureus 6538 were investigated by calculating the decreased ratio of cell death versus live HeLa cells. By further investigating the efficiency of the immune clearance of biotinylated S. aureus by mouse macrophages, we observed the enhanced uptake of S. aureus by murine macrophages in vivo. Overall, our work reveals that the biotinylation of AgrC can inhibit the growth and toxicity of S. aureus while simultaneously promoting the clearance of biotinylated S. aureus via macrophage phagocytosis.

Coupling Proximity Biotinylation with Genomic Targeting to Characterize Locus-Specific Changes in Chromatin Environments

Regulating gene expression involves significant changes in the chromatin environment at the locus level, especially at regulatory sequences. However, their modulation following pharmacological treatments or pathological conditions remain mostly undetermined. Here, we report versatile locus-specific proteomics tools to address this knowledge gap, which combine the targeting ability of the CRISPR/Cas9 system and the protein-labeling capability of the highly reactive biotin ligases TurboID (in CasTurbo) and UltraID (in CasUltra). CasTurbo and CasUltra enabled rapid chromatin protein labeling at repetitive sequences like centromeres and telomeres, as well as nonamplified genes. We applied CasUltra to A375 melanoma cell lines to decipher the protein environment of the MYC promoter and characterize the molecular effects of the bromodomain inhibitor JQ1, which targets bromodomain and extra-terminal (BET) proteins that regulate MYC expression. We quantified the consequences of BET protein displacement from the MYC promoter and found that it was associated with a considerable reorganization of the chromatin composition. Additionally, BET protein retention at the MYC promoter was consistent with a model of increased JQ1 resistance. Thus, through the combination of proximity biotinylation and CRISPR/Cas9 genomic targeting, CasTurbo and CasUltra have successfully demonstrated their utility in profiling the proteome associated with a genomic locus in living cells.

Retromer promotes the lysosomal turnover of mtDNA

Mitochondrial DNA (mtDNA) is exposed to multiple insults produced by normal cellular function. Upon mtDNA replication stress, the mitochondrial genome transfers to endosomes for degradation. Using proximity biotinylation, we found that mtDNA stress leads to the rewiring of the mitochondrial proximity proteome, increasing mitochondria's association with lysosomal and vesicle-related proteins. Among these, the retromer complex, particularly VPS35, plays a pivotal role by extracting mitochondrial components. The retromer promotes the formation of mitochondrial-derived vesicles shuttled to lysosomes. The mtDNA, however, directly shuttles to a recycling organelle in a BAX-dependent manner. Moreover, using a Drosophila model carrying a long deletion on the mtDNA (ΔmtDNA), we found that ΔmtDNA activates a specific transcriptome profile to counteract mitochondrial damage. Here, Vps35 expression restores mtDNA homoplasmy and alleviates associated defects. Hence, we demonstrate the existence of a previously unknown quality control mechanism for the mitochondrial matrix and the essential role of lysosomes in mtDNA turnover to relieve mtDNA damage.

Polycystins recruit cargo to distinct ciliary extracellular vesicle subtypes in C. elegans

Therapeutic use of tiny extracellular vesicles (EVs) requires understanding cargo loading mechanisms. Here, we use a modular proximity labeling approach to identify the cargo of ciliary EVs associated with the transient receptor potential channel polycystin-2 PKD-2 of C. elegans. Polycystins are conserved ciliary proteins and cargo of EVs; dysfunction causes polycystic kidney disease in humans and mating deficits in C. elegans. We discover that polycystins localize with specific cargo on ciliary EVs: polycystin-associated channel-like protein PACL-1, dorsal and ventral polycystin-associated membrane C-type lectins PAMLs, and conserved tumor necrosis factor receptor-associated factor (TRAF) TRF-1 and TRF-2. Loading of these components to EVs relies on polycystin-1 LOV-1. Our modular EV-TurboID approach can be applied in both cell- and tissue-specific manners to define the composition of distinct EV subtypes, addressing a major challenge of the EV field.

Identification of a molecular resistor that controls UCP1-independent Ca2+ cycling thermogenesis in adipose tissue

Adipose tissue thermogenesis contributes to energy balance via mitochondrial uncoupling protein 1 (UCP1) and UCP1-independent pathways. Among UCP1-independent thermogenic mechanisms, one involves Ca2+ cycling via SERCA2b in adipose tissue; however, the underlying molecular basis remains elusive. Here, we report that an endoplasmic reticulum (ER) membrane-anchored peptide, C4orf3 (also known as another regulin [ALN]), uncouples SERCA2b Ca2+ transport from its ATP hydrolysis, rendering the SERCA2b-C4orf3 complex exothermic. Loss of C4orf3/ALN improved the energetic efficiency of SERCA2b-dependent Ca2+ transport without affecting SERCA2 expression, thereby reducing adipose tissue thermogenesis and increasing the adiposity of mice. Notably, genetic depletion of C4orf3 resulted in compensatory activation of UCP1-dependent thermogenesis following cold challenge. We demonstrated that genetic loss of both C4orf3 and Ucp1 additively impaired cold tolerance in vivo. Together, this study identifies C4orf3 as the molecular resistor to SERCA2b-mediated Ca2+ import that plays a key role in UCP1-independent thermogenesis and energy balance.

Identification of E3 Ubiquitin Ligase Substrates Using Biotin Ligase-Based Proximity Labeling Approaches

Ubiquitylation is a post-translational modification originally identified as the first step in protein degradation by the ubiquitin-proteasome system. Ubiquitylation is also known to regulate many cellular processes without degrading the ubiquitylated proteins. Substrate proteins are specifically recognized and ubiquitylated by ubiquitin ligases. It is necessary to identify the substrates for each ubiquitin ligase to understand the physiological and pathological roles of ubiquitylation. Recently, a promiscuous mutant of a biotin ligase derived from Escherichia coli, BioID, and its variants have been utilized to analyze protein-protein interaction. In this review, we summarize the current knowledge regarding the molecular mechanisms underlying ubiquitylation, BioID-based approaches for interactome studies, and the application of BirA and its variants for the identification of ubiquitin ligase substrates.

Sex and Depot Specific Adipocyte Proteome Profiling In Vivo via Intracellular Proximity Labeling

Adipose tissue has varying distributions and metabolic properties between the sexes. Inherent sex-specific differences in adipocytes may heighten the risk of metabolic disease in males. Analysis of the adipocyte proteome can potentially provide important insight. To enable cell-type specific proteomic profiling in vivo, we genetically engineered a mouse line for cell-type specific production of a promiscuous biotin ligase (BirA*G3) facilitating the rapid isolation of biotinylated cell-type specific proteomes. Adipocyte-specific activation of cytoplasmic BirA*G3 led to robust biotinylation of adipocyte proteins across all major fat depots. Comparison of brown adipose tissue (BAT) and subcutaneous white adipose tissue (SAT) proteomes identified 229 brown adipose-enriched and 35 white adipose-enriched proteins. Regional comparison of white fat depots revealed additional differences across depots. Comparison of male and female depots identified sexually dimorphic adipose proteins: AHNAK predominating in the male and ACOT2 in the female. These findings validate the genetic model and highlight insights to be gained through targeted profiling of adipocytes. The genetic tool adds to existing approaches for in vivo proximity profiling of cell-type specific proteome programs.

PICH impacts the spindle assembly checkpoint via its DNA translocase and SUMO-interaction activities

Either inhibiting or stabilizing SUMOylation in mitosis causes defects in chromosome segregation, suggesting that dynamic mitotic SUMOylation of proteins is critical to maintain integrity of the genome. Polo-like kinase 1-interacting checkpoint helicase (PICH), a mitotic chromatin remodeling enzyme, interacts with SUMOylated chromosomal proteins via three SUMO-interacting motifs (SIMs) to control their association with chromosomes. Using cell lines with conditional PICH depletion/PICH replacement, we revealed mitotic defects associated with compromised PICH functions toward SUMOylated chromosomal proteins. Defects in either remodeling activity or SIMs of PICH delayed mitotic progression caused by activation of the spindle assembly checkpoint (SAC) indicated by extended duration of Mad1 foci at centromeres. Proteomics analysis of chromosomal SUMOylated proteins whose abundance is controlled by PICH activity identified candidate proteins to explain the SAC activation phenotype. Among the identified candidates, Bub1 kinetochore abundance is increased upon loss of PICH. Our results demonstrated a novel relationship between PICH and the SAC, where PICH directly or indirectly affects Bub1 association at the kinetochore and impacts SAC activity to control mitosis.

Implications of frequent hitter E3 ligases in targeted protein degradation screens

Targeted protein degradation (TPD) offers a promising approach for chemical probe and drug discovery that uses small molecules or biologics to direct proteins to the cellular machinery for destruction. Among the >600 human E3 ligases, CRBN and VHL have served as workhorses for ubiquitin-proteasome system-dependent TPD. Identification of additional E3 ligases capable of supporting TPD would unlock the full potential of this mechanism for both research and pharmaceutical applications. This perspective discusses recent strategies to expand the scope of TPD and the surprising convergence of these diverse screening efforts on a handful of E3 ligases, specifically DCAF16, DCAF11 and FBXO22. We speculate that a combination of properties, including superficial ligandability, potential for promiscuous substrate interactions and high occupancy in Cullin-RING complexes, may position these E3 ligases as 'low-hanging fruit' in TPD screens. We also discuss complementary approaches that might further expand the E3 ligase landscape supporting TPD.

Proximity interactome of alphavirus replicase component nsP3 includes proviral host factors eIF4G and AHNAK

All positive-strand RNA viruses replicate their genomes in association with modified intracellular membranes, inducing either membrane invaginations termed spherules, or double-membrane vesicles. Alphaviruses encode four non-structural proteins nsP1-nsP4, all of which are essential for RNA replication and spherule formation. To understand the host factors associated with the replication complex, we fused the efficient biotin ligase miniTurbo with Semliki Forest virus (SFV) nsP3, which is located on the cytoplasmic surface of the spherules. We characterized the proximal proteome of nsP3 in three cell lines, including cells unable to form stress granules, and identified >300 host proteins constituting the microenvironment of nsP3. These included all the nsPs, as well as several previously characterized nsP3 binding proteins. However, the majority of the identified interactors had no previously identified roles in alphavirus replication, including 39 of the top 50 interacting proteins. The most prominent biological processes involving the proximal proteins were nucleic acid metabolism, translational regulation, cytoskeletal rearrangement and membrane remodeling. siRNA silencing confirmed six novel proviral factors, USP10, AHNAK, eIF4G1, SH3GL1, XAB2 and ANKRD17, which are associated with distinct cellular functions. All of these except SH3GL1 were also important for the replication of chikungunya virus. We discovered that the small molecule 4E1RCat, which inhibits the interaction between the canonical translation initiation factors eIF4G and eIF4E, exhibits antiviral activity against SFV. Since the same molecule was previously found to inhibit coronaviruses, this suggest the possibility that translation initiation factors could be considered as targets for broadly acting antivirals.

Milestone Review: Unlocking the Proteomics of Glycine Receptor Complexes

Glycine receptors (GlyRs) are typically known for mediating inhibitory synaptic transmission within the spinal cord and brainstem, but they also have key roles in embryonic brain development, learning/memory, inflammatory pain sensitization, and rhythmic breathing. GlyR dysfunction has been implicated in multiple neurological disease states, including startle disease (GlyR α1β) and neurodevelopmental disorders (NDDs) including autism spectrum disorder (ASD), intellectual disability (ID), developmental delay (DD) and epilepsy (GlyR α2). However, GlyRs do not operate in isolation but depend upon stable and transient protein-protein interactions (PPIs) that influence synaptic localization, homeostasis, signaling pathways, and receptor function. Despite the affinity purification of GlyRs using the antagonist strychnine over four decades ago, we still have much to learn about native GlyR stoichiometry and accessory proteins. In contrast to other neurotransmitter receptors, < 20 potential GlyR interactors have been identified to date. These include some well-known proteins that are vital to inhibitory synapse function, such as the postsynaptic scaffolding protein gephyrin and the RhoGEF collybistin. However, the majority of known interactors either bind to the GlyR α1 and β subunits, or the binding partner in the GlyR complex is unknown. Several potential GlyR interactors are not found at inhibitory synapses and/or have no clear functional role. Moreover, other GlyR interactors are secondary interactors that bind indirectly, for example, via gephyrin. In this review, we provide a critical evaluation of known GlyR interacting proteins and methodological limitations to date. We also provide a road map for the use of innovative and emerging interaction proteomic techniques that will unlock the GlyR interactome. With the emergence of disease-associated missense mutations in the α1, α2 and β subunit intracellular domains in startle disease and NDDs, understanding the identity and roles of GlyR accessory proteins is vital in understanding GlyR function and dysfunction in health and disease.

Biased Signaling in G Protein-Coupled Receptors: Understanding the Biological Relevance and Tools for Probing Functionally Selective Ligands

Biased signaling has transformed pharmacology by revealing that receptors, particularly G protein-coupled receptors (GPCRs), can activate specific intracellular pathways selectively rather than uniformly. This discovery enables the development of targeted therapeutics that minimize side effects by precisely modulating receptor activity. Functionally selective ligands, which preferentially activate distinct signaling branches, have become essential tools for exploring receptor mechanisms and uncovering the complexities of GPCR signaling. These ligands help clarify receptor function in various physiological and pathological contexts, offering profound implications for therapeutic innovation. GPCRs, which mediate a wide range of cellular responses through coupling to G proteins and arrestins, are key pharmacological targets, with nearly a third of FDA-approved drugs acting on them. Recent advancements in biosensor development, multiplex assay platforms, and deep mutational scanning methods are improving our ability to define GPCR signaling, allowing for a better understanding of biased signaling pathways.

1000 fold Ultra-Photosensitized Fluorescent Protein Mimics Toward Photocatalytic Proximity Labeling and Proteomic Profiling Functions

Photosensitizing fluorescent proteins (FP) (e.g. KillerRed) have been shown not capable of photo-catalytic protein proximity labeling for downstream proteomic profiling applications. To acquire such a function, FP chromophores are engineered in a 12 × 12 combinatorial matrix of synthetic analoges, achieving up to 1000 fold enhancement of reactive oxygen species (ROS) production compared to the natural FPs. FP chromophores are shown with larger dipole moments exhibit higher ROS yield toward protein labeling. By conjugating the ultra-photosensitized FP chromophore to HaloTag (namely upsFP tag), its photo-catalytic protein proximity labeling function is demonstrated using nucleophilic amino substrates. Through photochemical characterizations, theoretical calculation, and tandem mass spectrometry, a radical-mediated labeling mechanism is revealed with expanded reactivity toward diverse protein residues via a type I photosensitization pathway. Finally, a proteomic profiling application is showcased using the upsFP tag to resolve the dynamic interactome variations upon TAR DNA-binding protein 43 (TDP43) phase separation and suborganellar translocation. Together, this work demonstrates three orders of magnitude ultra-photosensitization of fluorescent protein chromophore enables photocatalytic protein proximity labeling and profiling functions that are impractical for natural fluorescent proteins.