HSP90 and its R2TP/Prefoldin-like cochaperone are involved in the cytoplasmic assembly of RNA polymerase II

PDRG1 is essential for early plant development as a component of the prefoldin-like complex

p53 AND DNA DAMAGE-REGULATED GENE1 (PDRG1) is part of the prefoldin-like complex (PFDLc) in plants and animals. Whether PDRG1 acts primarily as a subunit of PFDLc or as an independent subunit is not known in any eukaryote. Here, we show that impairment of PDRG1 activity in Arabidopsis thaliana leads to embryonic lethality, as is the case for the other prefoldin-like proteins UXT and AtURI. The subunits of PFDLc are the main interactors of PDRG1 in vivo, and the interactomes of PDRG1, UXT, and AtURI show strong overlaps, including subunits of nuclear RNA polymerases and various complexes of the spliceosome. Our results show that PDRG1 plays an essential role in Arabidopsis mainly as a subunit of PFDLc.

The structure of the R2T complex reveals a different architecture from the related HSP90 cochaperone R2TP

The R2TP complex is a specialized HSP90 cochaperone essential for the maturation of macromolecular complexes such as RNAPII and TORC1. R2TP is formed by a hetero-hexameric ring of AAA-ATPases RuvBL1 and RuvBL2, which interact with RPAP3 and PIH1D1. Several R2TP-like complexes have been described, but these are less well characterized. Here, we identified, characterized and determined the cryo-electron microscopy (cryo-EM) structure of R2T from Arabidopsis thaliana, which lacks PIH1D1 and is probably the only form of the complex in seed plants. In contrast to R2TP, R2T is organized as two rings of AtRuvBL1-AtRuvBL2a interacting back-to-back, with one AtRPAP3 anchored per ring. AtRPAP3 has no effect on the ATPase activity of AtRuvBL1-AtRuvBL2a and binds with a different stoichiometry than in human R2TP. We show that the interaction of AtRPAP3 with AtRuvBL2a and AtHSP90 occurs via a conserved mechanism. However, the distinct architectures of R2T and R2TP suggest differences in their functions and mechanisms.

Heat shock protein 90 is a chaperone regulator of HIV-1 latency

An estimated 32 million people live with HIV-1 globally. Combined antiretroviral therapy suppresses viral replication but therapy interruption results in viral rebound from a latent reservoir mainly found in memory CD4+ T cells. Treatment is therefore lifelong and not curative. Eradication of this viral reservoir requires hematopoietic stem cell transplantation from hemizygous or homozygous ΔCCR5 donors, which is not broadly applicable. Alternative cure strategies include the pharmacological reactivation of latently infected cells to promote their immune-mediated clearance, or the induction of deep latency. HIV-1 latency is multifactorial and linked to the activation status of the infected CD4+ T cell. Hence to perturb latency, multiple pathways need to be simultaneously targeted without affecting CD4+ T cell function. Hsp90 has been shown to regulate HIV-1 latency, although knowledge on the pathways is limited. Because Hsp90 promotes the proper folding of numerous cellular proteins required for HIV-1 gene expression, we hypothesized that Hsp90 might be a master regulator of latency. We tested this hypothesis using a polyclonal Jurkat cell model of latency and ex-vivo latently infected primary CD4+ T cells. We found that, in the Jurkat model, Hsp90 is required for HIV-1 reactivation mediated by the T-cell receptor, phorbol esters, TNF-α, inhibition of FOXO-1, and agonists of TLR-7 and TLR-8. In primary cells, Hsp90 regulates HIV-1 gene expression induced by stimulation of the T-cell receptor or in the presence of IL-7/IL-15 or a FOXO-1 inhibitor. Chemical inhibition of Hsp90 abrogated activation of the NF-kB, NFAT and AP-1 signal transduction pathways. Within the CD4+ T cell population, CDRA45+ CCR7+ "naïve" and CD45RA- CCR7- "effector memory" cells were most sensitive to Hsp90 inhibition, which did not perturb their phenotype or activation state. Our results indicate that Hsp90 is a master regulator of HIV-1 latency that can potentially be targeted in cure strategies.

CTDP1 and RPB7 stabilize Pol II and permit reinitiation

The mechanisms governing the termination and subsequent reinitiation of RNA polymerase II (Pol II) remain poorly understood. Here we find that depletion of RPB7 leads to the destabilization of Pol II's largest subunit, RPB1. This destabilization is influenced by the loop regions of RPB7, CDK9, the C-terminal domain (CTD) of RPB1, and its linker region. The stabilization process of RPB1 is regulated by the E3 ubiquitin ligase Cullin 3. Additionally, RPB7 interacts with the phosphatase CTDP1, which is crucial for maintaining RPB1 stability. RPB7 is also vital for the reinitiation of Pol II, engages with RNA processing factors, and is localized to the RNA exit channel of the Pol II complex. The absence of RPB7 compromises RNA processing. We propose that RPB7 recruits CTDP1 to dephosphorylate Pol II, enhancing its stability and facilitating efficient reinitiation, adding an emerging dimension to transcriptional regulation.

RuvBL1/2 reduce toxic dipeptide repeat protein burden in multiple models of C9orf72-ALS/FTD

A G4C2 hexanucleotide repeat expansion in C9orf72 is the most common cause of amyotrophic lateral sclerosis and frontotemporal dementia (C9ALS/FTD). Bidirectional transcription and subsequent repeat-associated non-AUG (RAN) translation of sense and antisense transcripts leads to the formation of five dipeptide repeat (DPR) proteins. These DPRs are toxic in a wide range of cell and animal models. Therefore, decreasing RAN-DPRs may be of therapeutic benefit in the context of C9ALS/FTD. In this study, we found that C9ALS/FTD patients have reduced expression of the AAA+ family members RuvBL1 and RuvBL2, which have both been implicated in aggregate clearance. We report that overexpression of RuvBL1, but to a greater extent RuvBL2, reduced C9orf72-associated DPRs in a range of in vitro systems including cell lines, primary neurons from the C9-500 transgenic mouse model, and patient-derived iPSC motor neurons. In vivo, we further demonstrated that RuvBL2 overexpression and consequent DPR reduction in our Drosophila model was sufficient to rescue a number of DPR-related motor phenotypes. Thus, modulating RuvBL levels to reduce DPRs may be of therapeutic potential in C9ALS/FTD.

miR-1233-3p Inhibits Angiopoietin-1-Induced Endothelial Cell Survival, Migration, and Differentiation

Angiopoietin-1 (Ang-1) and its receptor Tie-2 promote vascular integrity and angiogenesis. MicroRNAs (miRNAs) are involved in the regulation of many cellular functions, including endothelial cell (EC) survival, proliferation, and differentiation. Several reports indicate that these effects of miRNAs on EC functions are mediated through the modulation of angiogenesis factor signaling including that of vascular endothelial growth factor (VEGF). To date, very little is known about the roles played by miRNAs in the signaling and angiogenesis promoted by the Ang-1-Tie-2 receptor axis. Our high-throughput screening of miRNAs regulated by Ang-1 exposure in human umbilical vein endothelial cells (HUVECs) has identified miR-1233-3p as a mature miRNA whose cellular levels are significantly downregulated in response to Ang-1 exposure. The expression of miR-1233-3p in these cells is also downregulated by other angiogenesis factors including VEGF, fibroblast growth factor 2 (FGF-2), transforming growth factor β (TGFβ), and angiopoietin-2 (Ang-2). The overexpression of miR-1233-3p in HUVECs using specific mimics significantly attenuated cell survival, migration, and capillary-like tube formation, and promoted apoptosis. Moreover, miR-1233-3p overexpression resulted in reversal of the anti-apoptotic, pro-migration, and pro-differentiation effects of Ang-1. Biotinylated miRNA pull-down assays showed that p53 and DNA damage-regulated 1 (PDRG1) is a direct target of miR-1233-3p in HUVECs. The exposure of HUVECs to Ang-1, angiopoietin-2 (Ang-2), fibroblast growth factor 2 (FGF2), vascular endothelial growth factor (VEGF), or transforming growth factor β (TGFβ) triggers the regulation of PDRG1 expression. This study highlights that miR-1233-3p exerts inhibitory effects on Ang-1-induced survival, migration, and the differentiation of cultured ECs.

A Portrait of Three Mammalian Bicistronic mRNA Transcripts, Derived from the Genes ASNSD1, SLC35A4, and MIEF1

Recent advances in functional genomics have allowed identification of thousands of translated short open reading frames (sORFs) in the 5' leaders of mammalian mRNA transcripts. While most sORFs are unlikely to encode functional proteins, a small number have been shown to have evolved as protein-coding genes. As a result, dozens of these sORFs have already been annotated as protein-coding ORFs. mRNAs that contain both a protein-coding sORF and an annotated coding sequence (CDS) are referred to as bicistronic transcripts. In this study, we focus on three genes - ASNSD1, SLC35A4, and MIEF1 - which give rise to bicistronic mRNAs. We discuss recent findings regarding functional investigation of the corresponding polypeptide products, as well as how their translation is regulated, and how this unusual genetic arrangement may have evolved.

Dosage suppressors of gpn2ts mutants and functional insights into the role of Gpn2 in budding yeast

Gpn2 is a highly conserved protein essential for the assembly of RNA polymerase II (RNAPII) in eukaryotic cells. Mutations in Gpn2, specifically Phe105Tyr and Leu164Pro, confer temperature sensitivity and significantly impair RNAPII assembly. Despite its crucial role, the complete range of Gpn2 functions remains to be elucidated. To further explore these functions, we conducted large-scale multicopy suppressor screening in budding yeast, aiming to identify genes whose overexpression could mitigate the growth defects of a temperature-sensitive gpn2 mutant (gpn2ts) at restrictive temperatures. We screened over 30,000 colonies harboring plasmids from a multicopy genetic library and identified 31 genes that rescued the growth defects of gpn2ts to various extents. Notably, we found that PAB1, CDC5, and RGS2 reduced the drug sensitivity of gpn2ts mutants. These findings lay a theoretical foundation for future studies on the function of Gpn2 in RNAPII assembly.

The HSP90/R2TP Quaternary Chaperone Scaffolds Assembly of the TSC Complex

The R2TP chaperone is composed of the RUVBL1/RUVBL2 AAA+ ATPases and two adapter proteins, RPAP3 and PIH1D1. Together with HSP90, it functions in the assembly of macromolecular complexes that are often involved in cell proliferation. Here, proteomic experiments using the isolated PIH domain reveals additional R2TP partners, including the Tuberous Sclerosis Complex (TSC) and many transcriptional complexes. The TSC is a key regulator of mTORC1 and is composed of TSC1, TSC2 and TBC1D7. We show a direct interaction of TSC1 with the PIH phospho-binding domain of PIH1D1, which is, surprisingly, phosphorylation independent. Via the use of mutants and KO cell lines, we observe that TSC2 makes independent interactions with HSP90 and the TPR domains of RPAP3. Moreover, inactivation of PIH1D1 or the RUVBL1/2 ATPase activity inhibits the association of TSC1 with TSC2. Taken together, these data suggest a model in which the R2TP recruits TSC1 via PIH1D1 and TSC2 via RPAP3 and HSP90, and use the chaperone-like activities of RUVBL1/2 to stimulate their assembly.

The nucleolar phase of signal recognition particle assembly

The signal recognition particle is essential for targeting transmembrane and secreted proteins to the endoplasmic reticulum. Remarkably, because they work together in the cytoplasm, the SRP and ribosomes are assembled in the same biomolecular condensate: the nucleolus. How important is the nucleolus for SRP assembly is not known. Using quantitative proteomics, we have investigated the interactomes of SRP components. We reveal that SRP proteins are associated with scores of nucleolar proteins important for ribosome biogenesis and nucleolar structure. Having monitored the subcellular distribution of SRP proteins upon controlled nucleolar disruption, we conclude that an intact organelle is required for their proper localization. Lastly, we have detected two SRP proteins in Cajal bodies, which indicates that previously undocumented steps of SRP assembly may occur in these bodies. This work highlights the importance of a structurally and functionally intact nucleolus for efficient SRP production and suggests that the biogenesis of SRP and ribosomes may be coordinated in the nucleolus by common assembly factors.

Maf1 phosphorylation is regulated through the action of prefoldin-like Bud27 on PP4 phosphatase in Saccharomyces cerevisiae

Bud27 is a prefoldin-like protein that participates in transcriptional regulation mediated by the three RNA polymerases in Saccharomyces cerevisiae. Lack of Bud27 significantly affects RNA pol III transcription, although the involved mechanisms have not been characterized. Here, we show that Bud27 regulates the phosphorylation state of the RNA pol III transcriptional repressor, Maf1, influences its nuclear localization, and likely its activity. We demonstrate that Bud27 is associated with the Maf1 main phosphatase PP4 in vivo, and that this interaction is required for proper Maf1 dephosphorylation. Lack of Bud27 decreases the interaction among PP4 and Maf1, Maf1 dephosphorylation, and its nuclear entry. Our data uncover a new nuclear function of Bud27, identify PP4 as a novel Bud27 interactor and demonstrate the effect of this prefoldin-like protein on the posttranslational regulation of Maf1. Finally, our data reveal a broader effect of Bud27 on PP4 activity by influencing, at least, the phosphorylation of Rad53.

Global control of RNA polymerase II

RNA polymerase II (Pol II) is the multi-protein complex responsible for transcribing all protein-coding messenger RNA (mRNA). Most research on gene regulation is focused on the mechanisms controlling which genes are transcribed when, or on the mechanics of transcription. How global Pol II activity is determined receives comparatively less attention. Here, we follow the life of a Pol II molecule from 'assembly of the complex' to nuclear import, enzymatic activity, and degradation. We focus on how Pol II spends its time in the nucleus, and on the two-way relationship between Pol II abundance and activity in the context of homeostasis and global transcriptional changes.

DIP-MS: ultra-deep interaction proteomics for the deconvolution of protein complexes

Most proteins are organized in macromolecular assemblies, which represent key functional units regulating and catalyzing most cellular processes. Affinity purification of the protein of interest combined with liquid chromatography coupled to tandem mass spectrometry (AP-MS) represents the method of choice to identify interacting proteins. The composition of complex isoforms concurrently present in the AP sample can, however, not be resolved from a single AP-MS experiment but requires computational inference from multiple time- and resource-intensive reciprocal AP-MS experiments. Here we introduce deep interactome profiling by mass spectrometry (DIP-MS), which combines AP with blue-native-PAGE separation, data-independent acquisition with mass spectrometry and deep-learning-based signal processing to resolve complex isoforms sharing the same bait protein in a single experiment. We applied DIP-MS to probe the organization of the human prefoldin family of complexes, resolving distinct prefoldin holo- and subcomplex variants, complex-complex interactions and complex isoforms with new subunits that were experimentally validated. Our results demonstrate that DIP-MS can reveal proteome modularity at unprecedented depth and resolution.

The association of the RSC remodeler complex with chromatin is influenced by the prefoldin-like Bud27 and determines nucleosome positioning and polyadenylation sites usage in Saccharomyces cerevisiae

The tripartite interaction between the chromatin remodeler complex RSC, RNA polymerase subunit Rpb5 and prefoldin-like Bud27 is necessary for proper RNA pol II elongation. Indeed lack of Bud27 alters this association and affects transcription elongation. This work investigates the consequences of lack of Bud27 on the chromatin association of RSC and RNA pol II, and on nucleosome positioning. Our results demonstrate that RSC binds chromatin in gene bodies and lack of Bud27 alters this association, mainly around polyA sites. This alteration impacts chromatin organization and leads to the accumulation of RNA pol II molecules around polyA sites, likely due to pausing or arrest. Our data suggest that RSC is necessary to maintain chromatin organization around those sites, and any alteration of this organization results in the widespread use of alternative polyA sites. Finally, we also find a similar molecular phenotype that occurs upon TOR inhibition with rapamycin, which suggests that alternative polyadenylation observed upon TOR inhibition is likely Bud27-dependent.

DPCD is a regulator of R2TP in ciliogenesis initiation through Akt signaling

R2TP is a chaperone complex consisting of the AAA+ ATPases RUVBL1 and RUVBL2, as well as RPAP3 and PIH1D1 proteins. R2TP is responsible for the assembly of macromolecular complexes mainly acting through different adaptors. Using proximity-labeling mass spectrometry, we identified deleted in primary ciliary dyskinesia (DPCD) as an adaptor of R2TP. Here, we demonstrate that R2TP-DPCD influences ciliogenesis initiation through a unique mechanism by interaction with Akt kinase to regulate its phosphorylation levels rather than its stability. We further show that DPCD is a heart-shaped monomeric protein with two domains. A highly conserved region in the cysteine- and histidine-rich domains-containing proteins and SGT1 (CS) domain of DPCD interacts with the RUVBL2 DII domain with high affinity to form a stable R2TP-DPCD complex both in cellulo and in vitro. Considering that DPCD is one among several CS-domain-containing proteins found to associate with RUVBL1/2, we propose that RUVBL1/2 are CS-domain-binding proteins that regulate complex assembly and downstream signaling.

Co-translational Assembly Pathways of Nuclear Multiprotein Complexes Involved in the Regulation of Gene Transcription

Most factors that regulate gene transcription in eukaryotic cells are multimeric, often large, protein complexes. The understanding of the biogenesis pathways of such large and heterogeneous protein assemblies, as well as the dimerization partner choice among transcription factors, is crucial to interpret and control gene expression programs and consequent cell fate decisions. Co-translational assembly (Co-TA) is thought to play key roles in the biogenesis of protein complexes by directing complex formation during protein synthesis. In this review we discuss the principles of Co-TA with a special focus for the assembly of transcription regulatory complexes. We outline the expected molecular advantages of establishing co-translational interactions, pointing at the available, or missing, evidence for each of them. We hypothesize different molecular mechanisms based on Co-TA to explain the allocation "dilemma" of paralog proteins and subunits shared by different transcription complexes. By taking as a paradigm the different assembly pathways employed by three related transcription regulatory complexes (TFIID, SAGA and ATAC), we discuss alternative Co-TA strategies for nuclear multiprotein complexes and the widespread - yet specific - use of Co-TA for the formation of nuclear complexes involved in gene transcription. Ultimately, we outlined a series of open questions which demand well-defined lines of research to investigate the principles of gene regulation that rely on the coordinated assembly of protein complexes.

Drugs targeting the particle for arrangement of quaternary structure (PAQosome) and protein complex assembly

INTRODUCTION

The PAQosome is a 12-subunit complex that acts as a co-factor of the molecular chaperones HSP90 and HSP70. This co-chaperone has been shown to participate in assembly and maturation of several protein complexes, including nuclear RNA polymerases, RNA processing factors, the ribosome, PIKKs, and others. Subunits of the PAQosome, adaptors, and clients have been reported to be involved in various diseases, making them interesting targets for drug discovery.

AREA COVERED

In this review, the authors cover the detailed mechanisms of PAQosome and chaperone function. Specifically, the authors summarize the status of the PAQosome and some related chaperones and co-chaperones as candidate targets for drug discovery. Indeed, a number of compounds are currently being tested for the development of treatments against diseases, such as cancers and neurodegenerative conditions.

EXPERT OPINION

Searching for new drugs targeting the PAQosome requires a better understanding of PAQosome subunit interactions and the discovery of new interaction partners. Thus, PAQosome subunit crystallization is an important experiment to initiate virtual screening against new target and the development of in silico tools such as AlphaFold-multimer could accelerate the search for new interaction partner and determine more rapidly the interaction pocket needed for virtual drug screening.

Maturation and Assembly of mTOR Complexes by the HSP90-R2TP-TTT Chaperone System: Molecular Insights and Mechanisms

The mechanistic target of rapamycin (mTOR) is a master regulator of cell growth and metabolism, integrating environmental signals to regulate anabolic and catabolic processes, regulating lipid synthesis, growth factor-induced cell proliferation, cell survival, and migration. These activities are performed as part of two distinct complexes, mTORC1 and mTORC2, each with specific roles. mTORC1 and mTORC2 are elaborated dimeric structures formed by the interaction of mTOR with specific partners. mTOR functions only as part of these large complexes, but their assembly and activation require a dedicated and sophisticated chaperone system. mTOR folding and assembly are temporarily separated with the TELO2-TTI1-TTI2 (TTT) complex assisting the cotranslational folding of mTOR into a native conformation. Matured mTOR is then transferred to the R2TP complex for assembly of active mTORC1 and mTORC2 complexes. R2TP works in concert with the HSP90 chaperone to promote the incorporation of additional subunits to mTOR and dimerization. This review summarizes our current knowledge on how the HSP90-R2TP-TTT chaperone system facilitates the maturation and assembly of active mTORC1 and mTORC2 complexes, discussing interactions, structures, and mechanisms.

Archaeal GPN-loop GTPases involve a lock-switch-rock mechanism for GTP hydrolysis

IMPORTANCE

GPN-loop GTPases have been found to be crucial for eukaryotic RNA polymerase II assembly and nuclear trafficking. Despite their ubiquitous occurrence in eukaryotes and archaea, the mechanism by which these GTPases mediate their function is unknown. Our study on an archaeal representative from Sulfolobus acidocaldarius showed that these dimeric GTPases undergo large-scale conformational changes upon GTP hydrolysis, which can be summarized as a lock-switch-rock mechanism. The observed requirement of SaGPN for motility appears to be due to its large footprint on the archaeal proteome.

The multi-faceted roles of R2TP complex span across regulation of gene expression, translation, and protein functional assembly

Macromolecular complexes play essential roles in various cellular processes. The assembly of macromolecular assemblies within the cell must overcome barriers imposed by a crowded cellular environment which is characterized by an estimated concentration of biological macromolecules amounting to 100-450 g/L that take up approximately 5-40% of the cytoplasmic volume. The formation of the macromolecular assemblies is facilitated by molecular chaperones in cooperation with their co-chaperones. The R2TP protein complex has emerged as a co-chaperone of Hsp90 that plays an important role in macromolecular assembly. The R2TP complex is composed of a heterodimer of RPAP3:P1H1DI that is in turn complexed to members of the ATPase associated with diverse cellular activities (AAA +), RUVBL1 and RUVBL2 (R1 and R2) families. What makes the R2TP co-chaperone complex particularly important is that it is involved in a wide variety of cellular processes including gene expression, translation, co-translational complex assembly, and posttranslational protein complex formation. The functional versatility of the R2TP co-chaperone complex makes it central to cellular development; hence, it is implicated in various human diseases. In addition, their roles in the development of infectious disease agents has become of interest. In the current review, we discuss the roles of these proteins as co-chaperones regulating Hsp90 and its partnership with Hsp70. Furthermore, we highlight the structure-function features of the individual proteins within the R2TP complex and describe their roles in various cellular processes.

Never a dull enzyme, RNA polymerase II

RNA polymerase II (Pol II) is composed of 12 subunits that collaborate to synthesize mRNA within the nucleus. Pol II is widely recognized as a passive holoenzyme, with the molecular functions of its subunits largely ignored. Recent studies employing auxin-inducible degron (AID) and multi-omics techniques have revealed that the functional diversity of Pol II is achieved through the differential contributions of its subunits to various transcriptional and post-transcriptional processes. By regulating these processes in a coordinated manner through its subunits, Pol II can optimize its activity for diverse biological functions. Here, we review recent progress in understanding Pol II subunits and their dysregulation in diseases, Pol II heterogeneity, Pol II clusters and the regulatory roles of RNA polymerases.

HSP90 and Noncoding RNAs

Heat shock protein 90 (HSP90) family is a class of proteins known as molecular chaperones that promote client protein folding and translocation in unstressed cells and regulate cellular homeostasis in the stress response. Noncoding RNAs (ncRNAs) are defined as RNAs that do not encode proteins. Previous studies have shown that ncRNAs are key regulators of multiple fundamental cellular processes, such as development, differentiation, proliferation, transcription, post-transcriptional modifications, apoptosis, and cell metabolism. It is known that ncRNAs do not act alone but function via the interactions with other molecules, including co-chaperones, RNAs, DNAs, and so on. As a kind of molecular chaperone, HSP90 is also involved in many biological procedures of ncRNAs. In this review, we systematically analyze the impact of HSP90 on various kinds of ncRNAs, including their synthesis and function, and how ncRNAs influence HSP90 directly and indirectly.

ATAC and SAGA co-activator complexes utilize co-translational assembly, but their cellular localization properties and functions are distinct

To understand the function of multisubunit complexes, it is of key importance to uncover the precise mechanisms that guide their assembly. Nascent proteins can find and bind their interaction partners during their translation, leading to co-translational assembly. Here, we demonstrate that the core modules of ATAC (ADA-two-A-containing) and SAGA (Spt-Ada-Gcn5-acetyltransferase), two lysine acetyl transferase-containing transcription co-activator complexes, assemble co-translationally in the cytoplasm of mammalian cells. In addition, a SAGA complex containing all of its modules forms in the cytoplasm and acetylates non-histone proteins. In contrast, ATAC complex subunits cannot be detected in the cytoplasm of mammalian cells. However, an endogenous ATAC complex containing two functional modules forms and functions in the nucleus. Thus, the two related co-activators, ATAC and SAGA, assemble using co-translational pathways, but their subcellular localization, cytoplasmic abundance, and functions are distinct.

Subcellular localization shapes the fate of RNA polymerase III

RNA polymerase III (Pol III) plays a vital role in transcription and as a viral-DNA sensor, but how it is assembled and distributed within cells remains poorly understood. Here, we show that Pol III is assembled with chaperones in the cytoplasm and forms transcription-dependent protein clusters upon transport into the nucleus. The largest subunit (RPC1) depletion through an auxin-inducible degron leads to rapid degradation and disassembly of Pol III complex in the nucleus and cytoplasm, respectively. This generates a pool of partially assembled Pol III intermediates, which can be rapidly mobilized into the nucleus upon the restoration of RPC1. Our study highlights the critical role of subcellular localization in determining Pol III's fate and provides insight into the dynamic regulation of nuclear Pol III levels and the origin of cytoplasmic Pol III complexes involved in mediating viral immunity.

Unveiling the Role of Sorghum RPAP3 in the Function of R2TP Complex: Insights into Protein Assembly in Plants

The chaperone R2TP has multiple subunits that assist in the proper folding, assembly, and stabilization of various protein complexes in cells and its study can offer valuable insights into the regulation and maintenance of protein assemblies in plant systems. The 'T' component of R2TP is Tah1 in yeast, consisting of 111 residues, while its counterpart in humans is RPAP3, with 665 residues. RPAP3 acts as a co-chaperone of Hsp90 and facilitates interactions between RUVBL proteins and other complex components, enhancing the recruitment of client proteins by the R2TP complex. These facts further underscore the relevance of studying this complex in different organisms. The putative gene corresponding to the RPAP3 in Sorghum bicolor, a monocotyledon plant, was cloned, and the protein (396 residues) purified for biochemical characterization. SbRPAP3 exists as a folded monomer and has a RPAP3 domain, which is present in human RPAP3 but absent in yeast Tah1. SbRPAP3 retains its functional capabilities, including binding with RUVBLs, Hsp90, and Hsp70. By elucidating the role of RPAP3 in plant R2TP complex, we can further comprehend the molecular mechanisms underlying plant-specific protein assembly and contribute to advancements in plant biology and biotechnological applications.

ATAC and SAGA coactivator complexes utilize co-translational assembly, but their cellular localization properties and functions are distinct

To understand the function of multisubunit complexes it is of key importance to uncover the precise mechanisms that guide their assembly. Nascent proteins can find and bind their interaction partners during their translation, leading to co-translational assembly. Here we demonstrate that the core modules of ATAC (ADA-Two-A-Containing) and SAGA (Spt-Ada-Gcn5-acetyltransferase), two lysine acetyl transferase-containing transcription coactivator complexes, assemble co-translationally in the cytoplasm of mammalian cells. In addition, SAGA complex containing all of its modules forms in the cytoplasm and acetylates non-histones proteins. In contrast, fully assembled ATAC complex cannot be detected in the cytoplasm of mammalian cells. However, endogenous ATAC complex containing two functional modules forms and functions in the nucleus. Thus, the two related coactivators, ATAC and SAGA, assemble by using co-translational pathways, but their subcellular localization, cytoplasmic abundance and functions are distinct.

RNA Pol II preferentially regulates ribosomal protein expression by trapping disassociated subunits

RNA polymerase II (RNA Pol II) has been recognized as a passively regulated multi-subunit holoenzyme. However, the extent to which RNA Pol II subunits might be important beyond the RNA Pol II complex remains unclear. Here, fractions containing disassociated RPB3 (dRPB3) were identified by size exclusion chromatography in various cells. Through a unique strategy, i.e., "specific degradation of disassociated subunits (SDDS)," we demonstrated that dRPB3 functions as a regulatory component of RNA Pol II to enable the preferential control of 3' end processing of ribosomal protein genes directly through its N-terminal domain. Machine learning analysis of large-scale genomic features revealed that the little elongation complex (LEC) helps to specialize the functions of dRPB3. Mechanistically, dRPB3 facilitates CBC-PCF11 axis activity to increase the efficiency of 3' end processing. Furthermore, RPB3 is dynamically regulated during development and diseases. These findings suggest that RNA Pol II gains specific regulatory functions by trapping disassociated subunits in mammalian cells.

A Workflow Guide to RNA-Seq Analysis of Chaperone Function and Beyond

RNA sequencing (RNA-seq) is a powerful method of transcriptional analysis that allows for the sequence identification and quantification of cellular transcripts. RNA-seq can be used for differential gene expression (DGE) analysis, gene fusion detection, allele-specific expression, isoform and splice variant quantification, and identification of novel genes. These applications can be used for downstream systems biology analyses such as gene ontology or pathway analysis to provide insight into processes altered between biological conditions. Given the wide range of signaling pathways subject to chaperone activity as well as numerous chaperone functions in RNA metabolism, RNA-seq may provide a valuable tool for the study of chaperone proteins in biology and disease. This chapter outlines an example RNA-seq workflow to determine differentially expressed (DE) genes between two or more sample conditions and provides some considerations for RNA-seq experimental design.

Maturation of small nucleolar RNAs: from production to function

Small Nucleolar RNAs (snoRNAs) are an abundant group of non-coding RNAs with well-defined roles in ribosomal RNA processing, folding and chemical modification. Besides their classic roles in ribosome biogenesis, snoRNAs are also implicated in several other cellular activities including regulation of splicing, transcription, RNA editing, cellular trafficking, and miRNA-like functions. Mature snoRNAs must undergo a series of processing steps tightly regulated by transiently associating factors and coordinated with other cellular processes including transcription and splicing. In addition to their mature forms, snoRNAs can contribute to gene expression regulation through their derivatives and degradation products. Here, we review the current knowledge on mechanisms of snoRNA maturation, including the different pathways of processing, and the regulatory mechanisms that control snoRNA levels and complex assembly. We also discuss the significance of studying snoRNA maturation, highlight the gaps in the current knowledge and suggest directions for future research in this area.

Chromatin Immunoprecipitation (ChIP) of Heat Shock Protein 90 (Hsp90)

Chromatin immunoprecipitation followed by sequencing (ChIP-seq) is a widely used technique for genome-wide mapping of protein-DNA interactions and epigenetic marks in vivo. Recent studies have suggested an important role of heat shock protein 90 (Hsp90) in chromatin. This molecular chaperone assists other proteins to acquire their mature and functional conformation and helps in the assembly of many complexes. In this chapter, we provide specific details on how to perform Hsp90 ChIP-seq from Drosophila Schneider (S2) cells. Briefly, cells are simultaneously lyzed and reversibly cross-linked to stabilize protein-DNA interactions. Chromatin is prepared from isolated nuclei and sheared by sonication. Hsp90-bound loci are immunoprecipitated and the corresponding DNA fragments are purified and sequenced. The described approach revealed that Hsp90 binds close to the transcriptional start site of around one-third of all Drosophila coding genes and characterized the role of the chaperone at chromatin.

Riluzole partially restores RNA polymerase III complex assembly in cells expressing the leukodystrophy-causative variant POLR3B R103H

The mechanism of assembly of RNA polymerase III (Pol III), the 17-subunit enzyme that synthesizes tRNAs, 5 S rRNA, and other small-nuclear (sn) RNAs in eukaryotes, is not clearly understood. The recent discovery of the HSP90 co-chaperone PAQosome (Particle for Arrangement of Quaternary structure) revealed a function for this machinery in the biogenesis of nuclear RNA polymerases. However, the connection between Pol III subunits and the PAQosome during the assembly process remains unexplored. Here, we report the development of a mass spectrometry-based assay that allows the characterization of Pol III assembly. This assay was used to dissect the stages of Pol III assembly, to start defining the function of the PAQosome in this process, to dissect the assembly defects driven by the leukodystrophy-causative R103H substitution in POLR3B, and to discover that riluzole, an FDA-approved drug for alleviation of ALS symptoms, partly corrects these assembly defects. Together, these results shed new light on the mechanism and regulation of human nuclear Pol III biogenesis.

Rtr1 is required for Rpb1-Rpb2 assembly of RNAPII and prevents their cytoplasmic clump formation

RNA polymerase II (RNAPII) is an essential machinery for catalyzing mRNA synthesis and controlling cell fate in eukaryotes. Although the structure and function of RNAPII have been relatively defined, the molecular mechanism of its assembly process is not clear. The identification and functional analysis of assembly factors will provide new understanding to transcription regulation. In this study, we identify that RTR1, a known transcription regulator, is a new multicopy genetic suppressor of mutants of assembly factors Gpn3, Gpn2, and Rba50. We demonstrate that Rtr1 is directly required to assemble the two largest subunits of RNAPII by coordinating with Gpn3 and Npa3. Deletion of RTR1 leads to cytoplasmic clumping of RNAPII subunit and multiple copies of RTR1 can inhibit the formation of cytoplasmic clump of RNAPII subunit in gpn3-9 mutant, indicating a new layer function of Rtr1 in checking proper assembly of RNAPII. In addition, we find that disrupted activity of Rtr1 phosphatase does not trigger the formation of cytoplasmic clump of RNAPII subunit in a catalytically inactive mutant of RTR1. Based on these results, we conclude that Rtr1 cooperates with Gpn3 and Npa3 to assemble RNAPII core.

The transcriptional coactivator RUVBL2 regulates Pol II clustering with diverse transcription factors

RNA polymerase II (Pol II) apparatuses are compartmentalized into transcriptional clusters. Whether protein factors control these clusters remains unknown. In this study, we find that the ATPase-associated with diverse cellular activities (AAA + ) ATPase RUVBL2 co-occupies promoters with Pol II and various transcription factors. RUVBL2 interacts with unphosphorylated Pol II in chromatin to promote RPB1 carboxy-terminal domain (CTD) clustering and transcription initiation. Rapid depletion of RUVBL2 leads to a decrease in the number of Pol II clusters and inhibits nascent RNA synthesis, and tethering RUVBL2 to an active promoter enhances Pol II clustering at the promoter. We also identify target genes that are directly linked to the RUVBL2-Pol II axis. Many of these genes are hallmarks of cancers and encode proteins with diverse cellular functions. Our results demonstrate an emerging activity for RUVBL2 in regulating Pol II cluster formation in the nucleus.

Targeted protein degradation reveals RNA Pol II heterogeneity and functional diversity

RNA polymerase II (RNA Pol II) subunits are thought to be involved in various transcription-associated processes, but it is unclear whether they play different regulatory roles in modulating gene expression. Here, we performed nascent and mature transcript sequencing after the acute degradation of 12 mammalian RNA Pol II subunits and profiled their genomic binding sites and protein interactomes to dissect their molecular functions. We found that RNA Pol II subunits contribute differently to RNA Pol II cellular localization and transcription processes and preferentially regulate RNA processing (such as RNA splicing and 3' end maturation). Genes sensitive to the depletion of different RNA Pol II subunits tend to be involved in diverse biological functions and show different RNA half-lives. Sequences, associated protein factors, and RNA structures are correlated with RNA Pol II subunit-mediated differential gene expression. These findings collectively suggest that the heterogeneity of RNA Pol II and different genes appear to depend on some of the subunits.

Macrophage Rmp Ameliorates Myocardial Infarction by Modulating Macrophage Polarization in Mice

Background

Inflammation plays important roles during myocardial infarction (MI). Macrophage polarization is a major factor that drives the inflammatory process. Our previous study found that RNA polymerase II subunit 5-mediating protein (RMP) knockout in cardiomyocytes caused heart failure by impairing mitochondrial structure and function. However, whether macrophage RMP plays a role in MI has not been investigated.

Methods

Macrophage RMP-knockout in combination with a mouse model of MI was used to study the function of macrophage RMP in MI. Next, we modified bone marrow-derived macrophages (BMDMs) by plasmid transfection, and the BMDMs were administered to LysM-Cre/DTR mice by tail vein injection. Immunoblotting and immunofluorescence were used to detect macrophage polarization, fibrosis, angiogenesis, and the p38 signaling pathway in each group.

Results

Macrophage RMP deficiency aggravates cardiac dysfunction, promotes M1 polarization, and inhibits angiogenesis after MI. However, RMP overexpression in macrophages promotes M2 polarization and angiogenesis after MI. Mechanistically, we found that RMP regulates macrophage polarization through the heat shock protein 90- (HSP90-) p38 signaling pathway.

Conclusions

Macrophage RMP plays a significant role in MI, likely by regulating macrophage polarization via the HSP90-p38 signaling pathway.

The Role of Hsp90-R2TP in Macromolecular Complex Assembly and Stabilization

Hsp90 is a ubiquitous molecular chaperone involved in many cell signaling pathways, and its interactions with specific chaperones and cochaperones determines which client proteins to fold. Hsp90 has been shown to be involved in the promotion and maintenance of proper protein complex assembly either alone or in association with other chaperones such as the R2TP chaperone complex. Hsp90-R2TP acts through several mechanisms, such as by controlling the transcription of protein complex subunits, stabilizing protein subcomplexes before their incorporation into the entire complex, and by recruiting adaptors that facilitate complex assembly. Despite its many roles in protein complex assembly, detailed mechanisms of how Hsp90-R2TP assembles protein complexes have yet to be determined, with most findings restricted to proteomic analyses and in vitro interactions. This review will discuss our current understanding of the function of Hsp90-R2TP in the assembly, stabilization, and activity of the following seven classes of protein complexes: L7Ae snoRNPs, spliceosome snRNPs, RNA polymerases, PIKKs, MRN, TSC, and axonemal dynein arms.

Development of a Methodology to Adapt an Equilibrium Buffer/Wash Applied to the Purification of hGPN2 Protein Expressed in Escherichia coli Using an IMAC Immobilized Metal Affinity Chromatography System

Protein purification is a complex and non-standardized process; the fact that proteins have different structural types making it difficult to create a standard methodology to obtain them in a pure, soluble, and homogeneous form. The present study shows the selective development of a buffer suitable for proteins of interest that allows high concentrations of hGPN2 protein to be obtained with low polydispersion and high homogeneity and purity. By taking the different reagents used in the construction of different buffers as a basis and performing purifications using different additives in different concentrations to determine the optimal amounts, the developed process helps to minimize the bonds, maintain solubility, release the proteins present in inclusion bodies, and provide an adequate environment for obtaining high concentrations of pure protein. GPN proteins are of unknown function, have not been purified in high concentrations, and have been found as part of the RNA polymerase assembly; if they are not expressed, the cell dies, and overexpression of certain GPN proteins has been linked to decreased survival in patients with invasive ductal carcinoma breast cancer types ER+ and HER2+. The results of the present study show that the use of the buffer developed for recombinant hGPN2 protein expressed in Escherichia coli could be manipulated in order to isolate the protein in a totally pure form and without the use of protease inhibitor tablets. The resulting homogeneity and low polydispersion was corroborated by studies carried out using dynamic dispersion analysis. Thanks to these properties, it can be used for crystallography or structural genomics studies.

Npa3-Gpn3 cooperate to assemble RNA polymerase II and prevent clump of its subunits in the cytoplasm

RNA polymerase II (RNAPII) is an essential machinery in eukaryotes that catalyzes mRNA synthesis and controls cell fate. Although the structure and function of RNAPII are relatively well defined, the molecular mechanism of its assembly process is poorly understood. Three members of GPN-loop GTPase family Npa3/Gpn1, Gpn2, and Gpn3 participate in the biogenesis of RNAPII with non-redundant roles. In this study, we demonstrate that Gpn3 and Npa3 directly participate in the assembly of the two largest subunits during biogenesis of RNAPII. When Gpn3 is defective, assembly of RNAPII is disrupted, leading to cytoplasmic foci of RNAPII subunits. Long-term assembly factor defects will lead to the accumulation of different kind of newly synthesized RNAPII subunits in the cytoplasm to form foci, and this can be prevented by recovery of the defective assembly factor. Cytoplasmic foci of RNAPII subunits in mutants of these assembly factors reveals a new cellular rescue response named the 'RNAPII assembly stress response'.

Rba50 and Gpn2 recruit the second largest subunits for the assembly of RNA polymerase II and III

Although remarkable progress has been made toward understanding the structures of eukaryotic RNA polymerases, the pathways and factors that facilitate their assembly remain unresolved. Essential proteins Rba50 and Gpn2 are required for Rpb3 subcomplex assembly, but whether they participate in subsequent assembly steps is unknown. Herein, we performed comprehensive genetic screens to explore Rba50 function. We identified two unique extragenic rba50-3-suppressing mutations that map to genes encoding the Rba50-interacting protein Gpn2, and Rpb2, the second largest subunit of RNAPII. Both gpn2-R347S and rpb2-V1171G variants bypass Rpb1 cytoplasmic arrest and temperature-sensitive growth defects of the rba50-3 mutant. GPN2 and RPB2 were also identified as novel multicopy suppressors of the rba50-3 mutant. Rapid depletion of Rba50 affected Rpb3-Rpb2 association during RNAPII assembly. Importantly, we demonstrated that Gpn2 facilitates the association of Rba50 and Rpb2. Our results imply that Rba50-Gpn2 interaction is essential for Rpb2 recruitment during RNAPII assembly following Rpb3 subcomplex assembly. Furthermore, the Rba50-Gpn2 complex appears to play a similar role in the assembly of RNAPIII. We therefore propose a model in which Rba50 interacts with Gpn2 and thereby promotes loading of the second largest subunit of RNAP II and III onto the previously assembled subcomplex.

The Association of Rpb4 with RNA Polymerase II Depends on CTD Ser5P Phosphatase Rtr1 and Influences mRNA Decay in Saccharomyces cerevisiae

Rtr1 is an RNA polymerase II (RNA pol II) CTD-phosphatase that influences gene expression during the transition from transcription initiation to elongation and during transcription termination. Rtr1 interacts with the RNA pol II and this interaction depends on the phosphorylation state of the CTD of Rpb1, which may influence dissociation of the heterodimer Rpb4/7 during transcription. In addition, Rtr1 was proposed as an RNA pol II import factor in RNA pol II biogenesis and participates in mRNA decay by autoregulating the turnover of its own mRNA. Our work shows that Rtr1 acts in RNA pol II assembly by mediating the Rpb4/7 association with the rest of the enzyme. RTR1 deletion alters RNA pol II assembly and increases the amount of RNA pol II associated with the chromatin that lacks Rpb4, decreasing Rpb4-mRNA imprinting and, consequently, increasing mRNA stability. Thus, Rtr1 interplays RNA pol II biogenesis and mRNA decay regulation. Our data also indicate that Rtr1 mediates mRNA decay regulation more broadly than previously proposed by cooperating with Rpb4. Interestingly, our data include new layers in the mechanisms of gene regulation and in the crosstalk between mRNA synthesis and decay by demonstrating how the association of Rpb4/7 to the RNA pol II influences mRNA decay.

Unphosphorylated Form of the PAQosome Core Subunit RPAP3 Binds Ribosomal Preassembly Complexes to Modulate Ribosome Biogenesis

The PAQosome (particle for arrangement of quaternary structure) is a 12-subunit HSP90 co-chaperone involved in the biogenesis of several human protein complexes. Two mechanisms of client selection have previously been identified, namely, the selective recruitment of specific adaptors and the differential use of homologous core subunits. Here, we describe a third client selection mechanism by showing that RPAP3, one of the core PAQosome subunits, is phosphorylated at several Ser residues in HEK293 cells. Affinity purification coupled with mass spectrometry (AP-MS) using the expression of tagged RPAP3 with single phospho-null mutations at Ser116, Ser119, or Ser121 reveals binding of the unphosphorylated form to several proteins involved in ribosome biogenesis. In vitro phosphorylation assays indicate that the kinase CK2 phosphorylates these RPAP3 residues. This finding is supported by data showing that pharmacological inhibition of CK2 enhances the binding of RPAP3 to ribosome preassembly factors in AP-MS experiments. Moreover, the silencing of PAQosome subunits interferes with ribosomal assembly factors' interactome. Altogether, these results indicate that RPAP3 phosphate group addition/removal at specific residues modulates binding to subunits of preribosomal complexes and allows speculating that PAQosome posttranslational modification is a mechanism of client selection.

Nuclear export restricts Gdown1 to a mitotic function

Approximately half of purified mammalian RNA polymerase II (Pol II) is associated with a tightly interacting sub-stoichiometric subunit, Gdown1. Previous studies have established that Gdown1 inhibits transcription initiation through competitive interactions with general transcription factors and blocks the Pol II termination activity of transcription termination factor 2 (TTF2). However, the biological functions of Gdown1 remain poorly understood. Here, we utilized genetic, microscopic, and multi-omics approaches to functionally characterize Gdown1 in three human cell lines. Acute depletion of Gdown1 caused minimal direct effects on transcription. We show that Gdown1 resides predominantly in the cytoplasm of interphase cells, shuttles between the cytoplasm and nucleus, and is regulated by nuclear export. Gdown1 enters the nucleus at the onset of mitosis. Consistently, genetic ablation of Gdown1 is associated with partial de-repression of mitotic transcription, and Gdown1 KO cells present with evidence of aberrant mitoses coupled to p53 pathway activation. Evidence is presented demonstrating that Gdown1 modulates the combined functions of purified productive elongation factors PAF1C, RTF1, SPT6, DSIF and P-TEFb in vitro. Collectively, our findings support a model wherein the Pol II-regulatory function of Gdown1 occurs during mitosis and is required for genome integrity.

Assembly principles of the human R2TP chaperone complex reveal the presence of R2T and R2P complexes

R2TP is a highly conserved chaperone complex formed by two AAA+ ATPases, RUVBL1 and RUVBL2, that associate with PIH1D1 and RPAP3 proteins. R2TP acts in promoting macromolecular complex formation. Here, we establish the principles of R2TP assembly. Three distinct RUVBL1/2-based complexes are identified: R2TP, RUVBL1/2-RPAP3 (R2T), and RUVBL1/2-PIH1D1 (R2P). Interestingly, we find that PIH1D1 does not bind to RUVBL1/RUVBL2 in R2TP and does not function as a nucleotide exchange factor; instead, RPAP3 is found to be the central subunit coordinating R2TP architecture and linking PIH1D1 and RUVBL1/2. We also report that RPAP3 contains an intrinsically disordered N-terminal domain mediating interactions with substrates whose sequences are primarily enriched for Armadillo repeat domains and other helical-type domains. Our work provides a clear and consistent model of R2TP complex structure and gives important insights into how a chaperone machine concerned with assembly of folded proteins into multisubunit complexes might work.

OUP accepted manuscript

MicroRNAs silence mRNAs by guiding the RISC complex. RISC assembly occurs following cleavage of pre-miRNAs by Dicer, assisted by TRBP or PACT, and the transfer of miRNAs to AGO proteins. The R2TP complex is an HSP90 co-chaperone involved in the assembly of ribonucleoprotein particles. Here, we show that the R2TP component RPAP3 binds TRBP but not PACT. The RPAP3-TPR1 domain interacts with the TRBP-dsRBD3, and the 1.5 Å resolution crystal structure of this complex identifies key residues involved in the interaction. Remarkably, binding of TRBP to RPAP3 or Dicer is mutually exclusive. Additionally, we found that AGO(1/2), TRBP and Dicer are all sensitive to HSP90 inhibition, and that TRBP sensitivity is increased in the absence of RPAP3. Finally, RPAP3 seems to impede miRNA activity, raising the possibility that the R2TP chaperone might sequester TRBP to regulate the miRNA pathway.

AGR2-Dependent Nuclear Import of RNA Polymerase II Constitutes a Specific Target of Pancreatic Ductal Adenocarcinoma in the Context of Wild-Type p53

BACKGROUND & AIMS

Promoted by pancreatitis, oncogenic KrasG12D triggers acinar cells' neoplastic transformation through acinar-to-ductal metaplasia (ADM) and pancreatic intraepithelial neoplasia. Anterior gradient 2 (Agr2), a known inhibitor of p53, is detected at early stage of pancreatic ductal adenocarcinoma (PDAC) development. RNA polymerase II (RNAPII) is a key nuclear enzyme; regulation of its nuclear localization in mammalian cells represents a potential therapeutic target.

METHODS

A mouse model of inflammation-accelerated KrasG12D-driven ADM and pancreatic intraepithelial neoplasia development was used. Pancreas-specific Agr2 ablation was performed to access its role in pancreatic carcinogenesis. Hydrophobic hexapeptides loaded in liposomes were developed to disrupt Agr2-RNAPII complex.

RESULTS

We found that Agr2 is up-regulated in ADM-to-pancreatic intraepithelial neoplasia transition in inflammation and KrasG12D-driven early pancreatic carcinogenesis. Genetic ablation of Agr2 specifically blocks this metaplastic-to-neoplastic process. Mechanistically, Agr2 directs the nuclear import of RNAPII via its C-terminal nuclear localization signal, undermining the ATR-dependent p53 activation in ADM lesions. Because Agr2 binds to the largest subunit of RNAPII in a peptide motif-dependent manner, we developed a hexapeptide to interfere with the nuclear import of RNAPII by competitively disrupting the Agr2-RNAPII complex. This novel hexapeptide leads to dysfunction of RNAPII with concomitant activation of DNA damage response in early neoplastic lesions; hence, it dramatically compromises PDAC initiation in vivo. Moreover, the hexapeptide sensitizes PDAC cells and patient-derived organoids harboring wild-type p53 to RNAPII inhibitors and first-line chemotherapeutic agents in vivo. Of note, this therapeutic effect is efficient across various cancer types.

CONCLUSIONS

Agr2 is identified as a novel adaptor protein for nuclear import of RNAPII in mammalian cells. Also, we provide genetic evidence defining Agr2-dependent nuclear import of RNAPII as a pharmaceutically accessible target for prevention and treatment in PDAC in the context of wild-type p53.

The HSP90/R2TP assembly chaperone promotes cell proliferation in the intestinal epithelium

The R2TP chaperone cooperates with HSP90 to integrate newly synthesized proteins into multi-subunit complexes, yet its role in tissue homeostasis is unknown. Here, we generated conditional, inducible knock-out mice for Rpap3 to inactivate this core component of R2TP in the intestinal epithelium. In adult mice, Rpap3 invalidation caused destruction of the small intestinal epithelium and death within 10 days. Levels of R2TP substrates decreased, with strong effects on mTOR, ATM and ATR. Proliferative stem cells and progenitors deficient for Rpap3 failed to import RNA polymerase II into the nucleus and they induced p53, cell cycle arrest and apoptosis. Post-mitotic, differentiated cells did not display these alterations, suggesting that R2TP clients are preferentially built in actively proliferating cells. In addition, high RPAP3 levels in colorectal tumors from patients correlate with bad prognosis. Here, we show that, in the intestine, the R2TP chaperone plays essential roles in normal and tumoral proliferation.

A nuclear proteome localization screen reveals the exquisite specificity of Gpn2 in RNA polymerase biogenesis

The GPN proteins are a conserved family of GTP-binding proteins that are involved in the assembly and subsequent import of RNA polymerase II and III. In this study, we sought to ascertain the specificity of yeast GPN2 for RNA polymerases by screening the localization of a collection of 1350 GFP-tagged nuclear proteins in WT or GPN2 mutant cells. We found that the strongest mislocalization occurred for RNA polymerase II and III subunits and only a handful of other RNAPII associated proteins were altered in GPN2 mutant cells. Our screen identified Ess1, an Rpb1 C-terminal domain (CTD) prolyl isomerase, as mislocalized in GPN2 mutants. Building on this observation we tested for effects of mutations in other factors which regulate Rpb1-CTD phosphorylation status. This uncovered significant changes in nuclear-cytoplasmic distribution of Rpb1-GFP in strains with disrupted RNA polymerase CTD kinases or phosphatases. Overall, this screen shows the exquisite specificity of GPN2 for RNA polymerase transport, and reveals a previously unappreciated role for CTD modification in RNAPII nuclear localization.

The Mammalian Ecdysoneless Protein Interacts with RNA Helicase DDX39A To Regulate Nuclear mRNA Export

The mammalian orthologue of ecdysoneless (ECD) protein is required for embryogenesis, cell cycle progression, and mitigation of endoplasmic reticulum stress. Here, we identified key components of the mRNA export complexes as binding partners of ECD and characterized the functional interaction of ECD with key mRNA export-related DEAD BOX protein helicase DDX39A. We find that ECD is involved in RNA export through its interaction with DDX39A. ECD knockdown (KD) blocks mRNA export from the nucleus to the cytoplasm, which is rescued by expression of full-length ECD but not an ECD mutant that is defective in interaction with DDX39A. We have previously shown that ECD protein is overexpressed in ErbB2⁺ breast cancers (BC). In this study, we extended the analyses to two publicly available BC mRNA The Cancer Genome Atlas (TCGA) and Molecular Taxonomy of Breast Cancer International Consortium (METABRIC) data sets. In both data sets, ECD mRNA overexpression correlated with short patient survival, specifically ErbB2⁺ BC. In the METABRIC data set, ECD overexpression also correlated with poor patient survival in triple-negative breast cancer (TNBC). Furthermore, ECD KD in ErbB2⁺ BC cells led to a decrease in ErbB2 mRNA level due to a block in its nuclear export and was associated with impairment of oncogenic traits. These findings provide novel mechanistic insight into the physiological and pathological functions of ECD.

The gamma-tubulin ring complex: Deciphering the molecular organization and assembly mechanism of a major vertebrate microtubule nucleator

Microtubules are protein cylinders with functions in cell motility, signal sensing, cell organization, intracellular transport, and chromosome segregation. One of the key properties of microtubules is their dynamic architecture, allowing them to grow and shrink in length by adding or removing copies of their basic subunit, the heterodimer αβ-tubulin. In higher eukaryotes, de novo assembly of microtubules from αβ-tubulin is initiated by a 2 MDa multi-subunit complex, the gamma-tubulin ring complex (γ-TuRC). For many years, the structure of the γ-TuRC and the function of its subunits remained enigmatic, although structural data from the much simpler yeast counterpart, the γ-tubulin small complex (γ-TuSC), were available. Two recent breakthroughs in the field, high-resolution structural analysis and recombinant reconstitution of the complex, have revolutionized our knowledge about the architecture and function of the γ-TuRC and will form the basis for addressing outstanding questions about biogenesis and regulation of this essential microtubule organizer.