LIRcentral: a manually curated online database of experimentally validated functional LIR motifs

TRIM16 mediates secretory autophagy in head and neck cancer-associated fibroblasts

Improving treatment options for head and neck squamous cell carcinoma (HNSCC) requires a deeper understanding of the tumor microenvironment, particularly cancer-associated fibroblasts (CAFs). We previously reported that HNSCC-derived FGF2/bFGF (fibroblast growth factor 2) triggers cytokine release from CAFs via secretory autophagy. Here, using transmission electron microscopy, live-cell imaging, and immunofluorescence, we show that CAF autophagosomes transport cargo, including IL6, to the plasma membrane for secretion. Autophagy in CAFs is constitutive and independent of STAT3, MAPK1/ERK2-MAPK3/ERK1 and phosphoinositide 3-kinase (PI3K) signaling. Despite the significant role of secretory autophagy in CAFs, its molecular machinery has remained elusive. Using both a literature based, and an unbiased approach, we studied the molecular machinery involved in autophagosome trafficking in CAFs. We identified TRIM16, a protein previously reported to traffic to autophagosomes, upregulated in CAFs compared to normal oral fibroblasts. Immunohistochemistry of patient HNSCC stroma revealed co-expression of TRIM16 and LC3B, linking TRIM16 to autophagosome function. An unbiased proteomics profiling of immunoprecipitated LC3B+ vesicles in primary HNSCC CAFs revealed enrichment in trafficking proteins, focal adhesion, and mitochondrial proteins. We demonstrate that SEC22B, SNAP23, VAMP3, and STX4 colocalize with LC3B, IL6, and TRIM16 in CAFs. TRIM16 knockdown reduced autophagosomes at the plasma membrane and decreased IL6 secretion from CAFs. These findings uncover key molecular components involved in autophagy-mediated IL6 secretion in CAFs and suggest potential therapeutic targets for HNSCC.Abbreviations: ACTA2/αSMA: actin alpha 2, smooth muscle; CAF: cancer-associated fibroblasts; CM: conditioned media; CQ: chloroquine; DAPI: 4',6-diamidino-2-phenylindole; DMSO: dimethylsulfoxide; EGFP: enhanced green fluorescent protein; ELISA: enzyme-linked immunosorbent assay; ER: endoplasmic reticulum; FGF2/bFGF: fibroblast growth factor 2; FGFR: fibroblast growth factor receptor; GO: gene ontology; GORASP2/GRASP55: golgi reassembly stacking protein 2; HMGB1: high mobility group box 1; HNSCC: head and neck squamous cell carcinoma; HPV: human papillomavirus; IL6: interleukin 6; IP: immunoprecipitation; LC-MS/MS: liquid chromatography-mass spectrometry/mass spectrometry; LIR: LC3-interacting region; MAP1LC3B/LC3B: microtubule associated protein 1 light chain 3 beta; MAPK1/ERK2: mitogen-activated protein kinase 1; MAPK3/ERK1: mitogen-activated protein kinase 3; NFs: normal oral fibroblasts; NSCLC: non-small cell lung cancer; PLA: proximity ligation assay; SQSTM1/p62: sequestosome 1; STAT3: signal transducer and activator of transcription 3; SNAP23: synaptosome associated protein 23; SNARE: soluble N-ethyl-maleimide-sensitive factor attachment protein receptor; STX4: syntaxin 4; TEM: transmission electron microscopy; TGFB1: transforming growth factor beta 1; TMA: tissue microarray; TRIM: tri-partite motif; VAMP: vesicle associated membrane protein; VC: vehicle control.

The LC3-interacting region of NBR1 is a protein interaction hub enabling optimal flux

During autophagy, toxic cargo is encapsulated by autophagosomes and trafficked to lysosomes for degradation. NBR1, an autophagy receptor targeting ubiquitinated aggregates, serves as a model for studying the multivalent, heterotypic interactions of cargo-bound receptors. Here, we find that three critical NBR1 partners-ATG8-family proteins, FIP200, and TAX1BP1-each bind to distinct, overlapping determinants within a short linear interaction motif (SLiM). To explore whether overlapping SLiMs extend beyond NBR1, we analyzed >100 LC3-interacting regions (LIRs), revealing that FIP200 and/or TAX1BP1 binding to LIRs is a common phenomenon and suggesting LIRs as protein interaction hotspots. Phosphomimetic peptides demonstrate that phosphorylation generally enhances FIP200 and ATG8-family binding but not TAX1BP1, indicating differential regulation. In vivo, LIR-mediated interactions with TAX1BP1 promote optimal NBR1 flux by leveraging additional functionalities from TAX1BP1. These findings reveal a one-to-many binding modality in the LIR motif of NBR1, illustrating the cooperative mechanisms of autophagy receptors and the regulatory potential of multifunctional SLiMs.

p62 sorts Lupus La and selected microRNAs into breast cancer-derived exosomes

Exosomes are multivesicular body-derived extracellular vesicles that are secreted by metazoan cells. Exosomes have utility as disease biomarkers, and exosome-mediated miRNA secretion has been proposed to facilitate tumor growth and metastasis. Previously, we demonstrated that the Lupus La protein (La) mediates the selective incorporation of miR-122 into metastatic breast cancer-derived exosomes; however, the mechanism by which La itself is sorted into exosomes remains unknown. Using unbiased proximity labeling proteomics, biochemical fractionation, superresolution microscopy and genetic tools, we establish that the selective autophagy receptor p62 sorts La and miR-122 into exosomes. We then performed small RNA sequencing and found that p62 depletion reduces the exosomal secretion of tumor suppressor miRNAs and results in their accumulation within cells. Our data indicate that p62 is a quality control factor that modulates the miRNA composition of exosomes. Cancer cells may exploit p62-dependent exosome cargo sorting to eliminate tumor suppressor miRNAs and thus to promote cell proliferation.

Cargo hitchhiking autophagy - a hybrid autophagy pathway utilized in yeast

Macroautophagy is a catabolic process that maintains cellular homeostasis by recycling intracellular material through the use of double-membrane vesicles called autophagosomes. In turn, autophagosomes fuse with vacuoles (in yeast and plants) or lysosomes (in metazoans), where resident hydrolases degrade the cargo. Given the conservation of autophagy, Saccharomyces cerevisiae is a valuable model organism for deciphering molecular details that define macroautophagy pathways. In yeast, macroautophagic pathways fall into two subclasses: selective and nonselective (bulk) autophagy. Bulk autophagy is predominantly upregulated following TORC1 inhibition, triggered by nutrient stress, and degrades superfluous random cytosolic proteins and organelles. In contrast, selective autophagy pathways maintain cellular homeostasis when TORC1 is active by degrading damaged organelles and dysfunctional proteins. Here, selective autophagy receptors mediate cargo delivery to the vacuole. Now, two groups have discovered a new hybrid autophagy mechanism, coined cargo hitchhiking autophagy (CHA), that uses autophagic receptor proteins to deliver selected cargo to phagophores built in response to nutrient stress for the random destruction of cytosolic contents. In CHA, various autophagic receptors link their cargos to lipidated Atg8, located on growing phagophores. In addition, the sorting nexin heterodimer Snx4-Atg20 assists in the degradation of cargo during CHA, possibly by aiding the delivery of cytoplasmic cargos to phagophores and/or by delaying the closure of expanding phagophores. This review will outline this new mechanism, also known as Snx4-assisted autophagy, that degrades an assortment of cargos in yeast, including transcription factors, glycogen, and a subset of ribosomal proteins.

NCOA4 initiates ferritinophagy by binding GATE16 using two highly avid short linear interaction motifs

Cells carefully regulate cytosolic iron, which is a vital enzymatic cofactor, yet is toxic in excess. In mammalian cells, surplus iron is sequestered in ferritin cages that, in iron limiting conditions, are degraded through the selective autophagy pathway ferritinophagy to liberate free iron. Prior work identified the ferritinophagy receptor protein NCOA4, which links ferritin and LC3/GABARAP-family member GATE16, effectively tethering ferritin to the autophagic machinery. Here, we elucidate the molecular mechanism underlying this interaction, discovering two short linear motifs in NCOA4 that each bind GATE16 with weak affinity. These binding motifs are highly avid and, in concert, support high-affinity NCOA4•GATE16 complex formation. We further find the minimal NCOA4383-522 fragment bearing these motifs is sufficient for ferritinophagy and that both motifs are necessary for this activity. This work suggests a general mechanism wherein selective autophagy receptors can distinguish between the inactive soluble pools of LC3/GABARAPs and the active membrane-conjugated forms that drive autophagy. Finally, we find that iron decreases NCOA4383-522's affinity for GATE16, providing a plausible mechanism for iron-dependent regulation of ferritinophagy.

The LC3-interacting region of NBR1 is a protein interaction hub enabling optimal flux

During autophagy, potentially toxic cargo is enveloped by a newly formed autophagosome and trafficked to the lysosome for degradation. Ubiquitinated protein aggregates, a key target for autophagy, are identified by multiple autophagy receptors. NBR1 is an archetypal autophagy receptor and an excellent model for deciphering the role of the multivalent, heterotypic interactions made by cargo-bound receptors. Using NBR1 as a model, we find that three critical binding partners - ATG8-family proteins, FIP200, and TAX1BP1 - each bind to a short linear interaction motif (SLiM) within NBR1. Mutational peptide arrays indicate that these binding events are mediated by distinct overlapping determinants, rather than a single, convergent, SLiM. AlphaFold modeling underlines the need for conformational flexibility within the NBR1 SLiM, as distinct conformations mediate each binding event. To test the extent to which overlapping SLiMs exist beyond NBR1, we performed peptide binding arrays on >100 established LC3-interacting regions (LIRs), revealing that FIP200 and/or TAX1BP1 binding to LIRs is a common phenomenon and suggesting LIRs as protein interaction hotspots. Comparative analysis of phosphomimetic peptides highlights that while FIP200 and Atg8-family binding are generally augmented by phosphorylation, TAX1BP1 binding is nonresponsive, suggesting differential regulation of these binding events. In vivo studies confirm that LIR-mediated interactions with TAX1BP1 enhance NBR1 activity, increasing autophagosomal delivery by leveraging an additional LIR from TAX1BP1. In sum, these results reveal a one-to-many binding modality in NBR1, providing key insights into the cooperative mechanisms among autophagy receptors. Furthermore, these findings underscore the pervasive role of multifunctional SLiMs in autophagy, offering substantial avenues for further exploration into their regulatory functions.

Linear motifs regulating protein secretion, sorting and autophagy in Leishmania parasites are diverged with respect to their host equivalents

The pathogenic, tropical Leishmania flagellates belong to an early-branching eukaryotic lineage (Kinetoplastida) with several unique features. Unfortunately, they are poorly understood from a molecular biology perspective, making development of mechanistically novel and selective drugs difficult. Here, we explore three functionally critical targeting short linear motif systems as well as their receptors in depth, using a combination of structural modeling, evolutionary sequence divergence and deep learning. Secretory signal peptides, endoplasmic reticulum (ER) retention motifs (KDEL motifs), and autophagy signals (motifs interacting with ATG8 family members) are ancient and essential components of cellular life. Although expected to be conserved amongst the kinetoplastids, we observe that all three systems show a varying degree of divergence from their better studied equivalents in animals, plants, or fungi. We not only describe their behaviour, but also build models that allow the prediction of localization and potential functions for several uncharacterized Leishmania proteins. The unusually Ala/Val-rich secretory signal peptides, endoplasmic reticulum resident proteins ending in Asp-Leu-COOH and atypical ATG8-like proteins are all unique molecular features of kinetoplastid parasites. Several of their critical protein-protein interactions could serve as targets of selective antimicrobial agents against Leishmaniasis due to their systematic divergence from the host.