SGTA recognizes a noncanonical ubiquitin-like domain in the Bag6-Ubl4A-Trc35 complex to promote endoplasmic reticulum-associated degradation

Structural and Functional Insights into Small, Glutamine-Rich, Tetratricopeptide Repeat Protein Alpha

The small glutamine-rich, tetratricopeptide repeat-containing protein alpha (SGTA) is an emerging player in the quality control of secretory and membrane proteins mislocalized to the cytosol, with established roles in tail-anchored (TA) membrane protein biogenesis. SGTA consists of three structural domains with individual functions, an N-terminal dimerization domain that assists protein sorting pathways, a central tetratricopeptide repeat (TPR) domain that mediates interactions with heat-shock proteins, proteasomal, and hormonal receptors, and viral proteins, and a C-terminal glutamine rich region that binds hydrophobic substrates. SGTA has been linked to viral lifecycles and hormone receptor signaling, with implications in the pathogenesis of various disease states. Thus far, a range of biophysical techniques have been employed to characterize SGTA structure in some detail, and to investigate its interactions with binding partners in different biological contexts. A complete description of SGTA structure, together with further investigation into its function as a co-chaperone involved quality control, could provide us with useful insights into its role in maintaining cellular proteostasis, and broaden our understanding of mechanisms underlying associated pathologies. This review describes how some structural features of SGTA have been elucidated, and what this has uncovered about its cellular functions. A brief background on the structure and function of SGTA is given, highlighting its importance to biomedicine and related fields. The current level of knowledge and what remains to be understood about the structure and function of SGTA is summarized, discussing the potential direction of future research.

Target Selection during Protein Quality Control

Protein quality control (QC) pathways survey the cellular proteome to selectively recognize and degrade faulty proteins whose accumulation can lead to various diseases. Recognition of the occasional aberrant protein among an abundant sea of similar normal counterparts poses a considerable challenge to the cell. Solving this problem requires protein QC machinery to assay multiple molecular criteria within a spatial and temporal context. Although each QC pathway has unique criteria and mechanisms for distinguishing right from wrong, they appear to share several general concepts. We discuss principles of high-fidelity target recognition, the decisive event of all protein QC pathways, to guide future work in this area.

A bacterial toxin and a nonenveloped virus hijack ER-to-cytosol membrane translocation pathways to cause disease

A dedicated network of cellular factors ensures that proteins translocated into the endoplasmic reticulum (ER) are folded correctly before they exit this compartment en route to other cellular destinations or for secretion. When proteins misfold, selective ER-resident enzymes and chaperones are recruited to rectify the protein-misfolding problem in order to maintain cellular proteostasis. However, when a protein becomes terminally misfolded, it is ejected into the cytosol and degraded by the proteasome via a pathway called ER-associated degradation (ERAD). Strikingly, toxins and viruses can hijack elements of the ERAD pathway to access the host cytosol and cause infection. This review focuses on emerging data illuminating the molecular mechanisms by which these toxic agents co-opt the ER-to-cytosol translocation process to cause disease.

Ubl4A is required for insulin-induced Akt plasma membrane translocation through promotion of Arp2/3-dependent actin branching

The serine-threonine kinase Akt is a key regulator of cell proliferation and survival, glucose metabolism, cell mobility, and tumorigenesis. Activation of Akt by extracellular stimuli such as insulin centers on the interaction of Akt with PIP3 on the plasma membrane, where it is subsequently phosphorylated and activated by upstream protein kinases. However, it is not known how Akt is recruited to the plasma membrane upon stimulation. Here we report that ubiquitin-like protein 4A (Ubl4A) plays a crucial role in insulin-induced Akt plasma membrane translocation. Ubl4A knockout newborn mice have defective Akt-dependent glycogen synthesis and increased neonatal mortality. Loss of Ubl4A results in the impairment of insulin-induced Akt translocation to the plasma membrane and activation. Akt binds actin-filaments and colocalizes with actin-related protein 2 and 3 (Arp2/3) complex in the membrane ruffles and lamellipodia. Ubl4A directly interacts with Arp2/3 to accelerate actin branching and networking, allowing Akt to be in close proximity to the plasma membrane for activation upon insulin stimulation. Our finding reveals a new mechanism by which Akt is recruited to the plasma membrane for activation, thereby providing a missing link in Akt signaling.

Binding of SGTA to Rpn13 selectively modulates protein quality control

Rpn13 is an intrinsic ubiquitin receptor of the 26S proteasome regulatory subunit that facilitates substrate capture prior to degradation. Here we show that the C-terminal region of Rpn13 binds to the tetratricopeptide repeat (TPR) domain of SGTA, a cytosolic factor implicated in the quality control of mislocalised membrane proteins (MLPs). The overexpression of SGTA results in a substantial increase in steady-state MLP levels, consistent with an effect on proteasomal degradation. However, this effect is strongly dependent upon the interaction of SGTA with the proteasomal component Rpn13. Hence, overexpression of the SGTA-binding region of Rpn13 or point mutations within the SGTA TPR domain both inhibit SGTA binding to the proteasome and substantially reduce MLP levels. These findings suggest that SGTA can regulate the access of MLPs to the proteolytic core of the proteasome, implying that a protein quality control cycle that involves SGTA and the BAG6 complex can operate at the 19S regulatory particle. We speculate that the binding of SGTA to Rpn13 enables specific polypeptides to escape proteasomal degradation and/or selectively modulates substrate degradation.

Highlighted Article: Binding of SGTA to the proteasome delays substrate degradation, thereby providing a mechanism for potentially viable proteins to be rescued for reuse.

Prohibitin 2: At a communications crossroads

Prohibitins (PHBs) are a highly conserved class of proteins first discovered as inhibitors of cellular proliferation. Since then PHBs have been found to have a significant role in transcription, nuclear signaling, mitochondrial structural integrity, cell division, and cellular membrane metabolism, placing these proteins among the key regulators of pathologies such as cancer, neuromuscular degeneration, and other metabolic diseases. The human genome encodes two PHB proteins, prohibitin 1 (PHB1) and prohibitin 2 (PHB2), which function not only as a heterodimeric complex, but also independently. While many previous reviews have focused on the better characterized prohibitin, PHB1, this review focuses on PHB2 and new data concerning its cellular functions both in complex with PHB1 and independent of PHB1.

Structure of a BAG6 (Bcl-2-associated athanogene 6)-Ubl4a (ubiquitin-like protein 4a) complex reveals a novel binding interface that functions in tail-anchored protein biogenesis

BAG6 is an essential protein that functions in two distinct biological pathways, ubiquitin-mediated protein degradation of defective polypeptides and tail-anchored (TA) transmembrane protein biogenesis in mammals, although its structural and functional properties remain unknown. We solved a crystal structure of the C-terminal heterodimerization domains of BAG6 and Ubl4a and characterized their interaction biochemically. Unexpectedly, the specificity and structure of the C terminus of BAG6, which was previously classified as a BAG domain, were completely distinct from those of the canonical BAG domain. Furthermore, the tight association of BAG6 and Ubl4a resulted in modulation of Ubl4a protein stability in cells. Therefore, we propose to designate the Ubl4a-binding region of BAG6 as the novel BAG-similar (BAGS) domain. The structure of Ubl4a, which interacts with BAG6, is similar to the yeast homologue Get5, which forms a homodimer. These observations indicate that the BAGS domain of BAG6 promotes the TA protein biogenesis pathway in mammals by the interaction with Ubl4a.

Bag6 complex contains a minimal tail-anchor-targeting module and a mock BAG domain

BCL2-associated athanogene cochaperone 6 (Bag6) plays a central role in cellular homeostasis in a diverse array of processes and is part of the heterotrimeric Bag6 complex, which also includes ubiquitin-like 4A (Ubl4A) and transmembrane domain recognition complex 35 (TRC35). This complex recently has been shown to be important in the TRC pathway, the mislocalized protein degradation pathway, and the endoplasmic reticulum-associated degradation pathway. Here we define the architecture of the Bag6 complex, demonstrating that both TRC35 and Ubl4A have distinct C-terminal binding sites on Bag6 defining a minimal Bag6 complex. A crystal structure of the Bag6-Ubl4A dimer demonstrates that Bag6-BAG is not a canonical BAG domain, and this finding is substantiated biochemically. Remarkably, the minimal Bag6 complex defined here facilitates tail-anchored substrate transfer from small glutamine-rich tetratricopeptide repeat-containing protein α to TRC40. These findings provide structural insight into the complex network of proteins coordinated by Bag6.

Solution structure of the SGTA dimerisation domain and investigation of its interactions with the ubiquitin-like domains of BAG6 and UBL4A

Background

The BAG6 complex resides in the cytosol and acts as a sorting point to target diverse hydrophobic protein substrates along their appropriate paths, including proteasomal degradation and ER membrane insertion. Composed of a trimeric complex of BAG6, TRC35 and UBL4A, the BAG6 complex is closely associated with SGTA, a co-chaperone from which it can obtain hydrophobic substrates.

Methodology and Principal Findings

SGTA consists of an N-terminal dimerisation domain (SGTA_NT), a central tetratricopeptide repeat (TPR) domain, and a glutamine rich region towards the C-terminus. Here we solve a solution structure of the SGTA dimerisation domain and use biophysical techniques to investigate its interaction with two different UBL domains from the BAG6 complex. The SGTA_NT structure is a dimer with a tight hydrophobic interface connecting two sets of four alpha helices. Using a combination of NMR chemical shift perturbation, isothermal titration calorimetry (ITC) and microscale thermophoresis (MST) experiments we have biochemically characterised the interactions of SGTA with components of the BAG6 complex, the ubiquitin-like domain (UBL) containing proteins UBL4A and BAG6. We demonstrate that the UBL domains from UBL4A and BAG6 directly compete for binding to SGTA at the same site. Using a combination of structural and interaction data we have implemented the HADDOCK protein-protein interaction docking tool to generate models of the SGTA-UBL complexes.

Significance

This atomic level information contributes to our understanding of the way in which hydrophobic proteins have their fate decided by the collaboration between SGTA and the BAG6 complex.

Structural and functional characterization of ybr137wp implicates its involvement in the targeting of tail-anchored proteins to membranes

Nearly 5% of membrane proteins are guided to nuclear, endoplasmic reticulum (ER), mitochondrial, Golgi, or peroxisome membranes by their C-terminal transmembrane domain and are classified as tail-anchored (TA) membrane proteins. In Saccharomyces cerevisiae, the guided entry of TA protein (GET) pathway has been shown to function in the delivery of TA proteins to the ER. The sorting complex for this pathway is comprised of Sgt2, Get4, and Get5 and facilitates the loading of nascent tail-anchored proteins onto the Get3 ATPase. Multiple pulldown assays also indicated that Ybr137wp associates with this complex in vivo. Here, we report a 2.8-Å-resolution crystal structure for Ybr137wp from Saccharomyces cerevisiae. The protein is a decamer in the crystal and also in solution, as observed by size exclusion chromatography and analytical ultracentrifugation. In addition, isothermal titration calorimetry indicated that the C-terminal acidic motif of Ybr137wp interacts with the tetratricopeptide repeat (TPR) domain of Sgt2. Moreover, an in vivo study demonstrated that Ybr137wp is induced in yeast exiting the log phase and ameliorates the defect of TA protein delivery and cell viability derived by the impaired GET system under starvation conditions. Therefore, this study suggests a possible role for Ybr137wp related to targeting of tail-anchored proteins.

SGTA regulates the cytosolic quality control of hydrophobic substrates

Hydrophobic amino acids are normally shielded from the cytosol and their exposure is often used as an indicator of protein misfolding to enable the chaperone-mediated recognition and quality control of aberrant polypeptides. Mislocalised membrane proteins (MLPs) represent a particular challenge to cellular quality control, and, in this study, membrane protein fragments have been exploited to study a specialised pathway that underlies the efficient detection and proteasomal degradation of MLPs. Our data show that the BAG6 complex and SGTA compete for cytosolic MLPs by recognition of their exposed hydrophobicity, and the data suggest that SGTA acts to maintain these substrates in a non-ubiquitylated state. Hence, SGTA might counter the actions of BAG6 to delay the ubiquitylation of specific precursors and thereby increase their opportunity for successful post-translational delivery to the endoplasmic reticulum. However, when SGTA is overexpressed, the normally efficient removal of aberrant MLPs is delayed, increasing their steady-state level and promoting aggregation. Our data suggest that SGTA regulates the cellular fate of a range of hydrophobic polypeptides should they become exposed to the cytosol.

Overexpression of BAT3 alleviates prion protein fragment PrP106-126-induced neuronal apoptosis

BACKGROUNDS AND AIMS

Prion diseases are a group of infectious neurodegenerative diseases characterized by neuronal death and degeneration. Human leukocyte antigen-B-associated transcript 3 (BAT3) is an important apoptosis regulator. We therefore investigated the interactions between BAT3 and prion protein and the potential role of BAT3 in PrP106-126-induced apoptosis.

METHODS

BAT3 and prion protein were overexpressed in Hela, Neuro2A, or primary neuronal cells by transfection with BAT3-HA or PRNP-EGFP expression plasmids and their relationship studied by immunofluorescence and Western blotting. The effect of BAT3 on PrP106-126-induced cytotoxicity and apoptosis was detected by the CCK-8 assay and terminal-deoxynucleotidyl transferase-mediated nick end labeling (TUNEL) assay. The expression of cytochrome c and Bcl-2 was examined by Western blotting.

RESULTS

BAT3 interacted with prion protein and enhanced PrP expression. After PrP106-126 peptide treated, BAT3 was transported from the nucleus to cytoplasm, increased cell viability, and protected neurons from PrP106-126-induced apoptosis through stabilizing the level of Bcl-2 protein and inhibiting the release of cytochrome c to cytoplasm.

CONCLUSIONS

Our present data showed a novel molecular mechanism of PrP106-126-induced apoptotic process regulation through the overexpression of BAT3, which may be important for the basic regulatory mechanism of neuron survival in prion diseases and associated neurodegenerative diseases in vivo.

Cleaning up in the endoplasmic reticulum: ubiquitin in charge

The eukaryotic endoplasmic reticulum (ER) maintains protein homeostasis by eliminating unwanted proteins through the evolutionarily conserved ER-associated degradation (ERAD) pathway. During ERAD, maturation-defective and surplus polypeptides are evicted from the ER lumen and/or lipid bilayer through the process of retrotranslocation and ultimately degraded by the proteasome. An integral facet of the ERAD mechanism is the ubiquitin system, composed of the ubiquitin modifier and the factors for assembling, processing and binding ubiquitin chains on conjugated substrates. Beyond simply marking polypeptides for degradation, the ubiquitin system is functionally intertwined with retrotranslocation machinery to transport polypeptides across the ER membrane.

Quality control: ER-associated degradation: protein quality control and beyond

Even with the assistance of many cellular factors, a significant fraction of newly synthesized proteins ends up misfolded. Cells evolved protein quality control systems to ensure that these potentially toxic species are detected and eliminated. The best characterized of these pathways, the ER-associated protein degradation (ERAD), monitors the folding of membrane and secretory proteins whose biogenesis takes place in the endoplasmic reticulum (ER). There is also increasing evidence that ERAD controls other ER-related functions through regulated degradation of certain folded ER proteins, further highlighting the role of ERAD in cellular homeostasis.

The final moments of misfolded proteins en route to the proteasome

Protein homeostasis in the endoplasmic reticulum (ER) in eukaryotic cells is maintained by a conserved quality control system named ER-associated degradation (ERAD). The ERAD system retains misfolded or unassembled polypeptides in the ER, retrotranslocates them into the cytosol for degradation by the ubiquitin proteasome system. Central to the ERAD process is the AAA+ (ATPase associated with various cellular activities), ATPase p97/VCP (also known as Cdc48p in yeast), and the proteasome. p97/VCP couples ATP hydrolysis to the extraction of misfolded proteins from retrotranslocation sites and subsequently targets them for degradation, but how p97/VCP hands substrate off to the proteasome is unclear. Recent studies suggest that p97/VCP may either directly translocate polypeptides into the proteolytic compartment of the 20S subcomplex, or use a set of shuttling factors to deliver retrotranslocated polypeptides to the proteasome.

BAG6 regulates the quality control of a polytopic ERAD substrate

BAG6 participates in protein quality control and, here, we address its role in endoplasmic-reticulum-associated degradation (ERAD) by using the polytopic membrane protein OpD, an opsin degron mutant. Both BAG6 knockdown and BAG6 overexpression delay OpD degradation; however, our data suggest that these two perturbations are mechanistically distinct. Hence, BAG6 knockdown correlates with reduced OpD polyubiquitylation, whereas BAG6 overexpression increases the level of polyubiquitylated OpD. The UBL- and BAG-domains of exogenous BAG6 are dispensable for OpD stabilisation and enhanced levels of polyubiquitylated OpD. Thus, although endogenous BAG6 normally promotes OpD degradation, exogenous BAG6 expression delays this process. We speculate that overexpressed BAG6 subunits might associate with the endogenous BAG6 complex, resulting in a dominant-negative effect that inhibits its function. Interestingly, cellular levels of BAG6 also correlate with total steady-state polyubiquitylation, with Rpn10 (officially known as PSMD4) overexpression showing a similar effect. These findings suggest that perturbations of the levels of ubiquitin-binding proteins can impact upon cellular ubiquitin homeostasis. We propose that exogenous BAG6 perturbs the function of the BAG6 complex at a stage subsequent to substrate recognition and polyubiquitylation, most likely the BAG6-dependent delivery of OpD to the proteasome.

BAG-6, a jack of all trades in health and disease

BCL2-associated athanogene 6 (BAG-6) (also Bat-3/Scythe) was discovered as a gene product of the major histocompatibility complex class III locus. The Xenopus ortholog Scythe was first identified to act as an anti-apoptotic protein. Subsequent studies unraveled that the large BAG-6 protein contributes to a number of cellular processes, including apoptosis, gene regulation, protein synthesis, protein quality control, and protein degradation. In this context, BAG-6 acts as a multifunctional chaperone, which interacts with its target proteins for shuttling to distinct destinations. Nonetheless, as anticipated from its genomic localization, BAG-6 is involved in a variety of immunological pathways such as macrophage function and TH1 response. Most recently, BAG-6 was identified on the plasma membrane of dendritic cells and malignantly transformed cells where it serves as cellular ligand for the activating natural killer (NK) cell receptor NKp30 triggering NK cell cytotoxicity. Moreover, target cells were found to secrete soluble variants of BAG-6 and release BAG-6 on the surface of exosomes, which inhibit or activate NK cell cytotoxicity, respectively. These data suggest that the BAG-6 antigen is an important target to shape a directed immune response or to overcome tumor-immune escape strategies established by soluble BAG-6. This review summarizes the currently known functions of BAG-6, a fascinating multicompetent protein, in health and disease.

The cochaperone SGTA (small glutamine-rich tetratricopeptide repeat-containing protein alpha) demonstrates regulatory specificity for the androgen, glucocorticoid, and progesterone receptors

Steroid hormone receptors are ligand-dependent transcription factors that require the ordered assembly of multichaperone complexes for transcriptional activity. Although heat shock protein (Hsp) 90 and Hsp70 are key players in this process, multiple Hsp70- and Hsp90-associated cochaperones associate with receptor-chaperone complexes to regulate receptor folding and activation. Small glutamine-rich tetratricopeptide repeat-containing protein alpha (SGTA) was recently characterized as an Hsp70 and Hsp90-associated cochaperone that specifically regulates androgen receptor activity. However, the specificity of SGTA for additional members of the steroid hormone receptor superfamily and the mechanism by which SGTA regulates receptor activity remain unclear. Here we report that SGTA associates with and specifically regulates the androgen, glucocorticoid, and progesterone receptors and has no effect on the mineralocorticoid and estrogen receptors in both yeast and mammalian cell-based reporter assays. In both systems, SGTA knockdown/deletion enhances receptor activity, whereas SGTA overexpression suppresses receptor activity. We demonstrate that SGTA binds directly to Hsp70 and Hsp90 in vitro with similar affinities yet predominately precipitates with Hsp70 from cell lysates, suggesting a role for SGTA in early, Hsp70-mediated folding. Furthermore, SGTA expression completely abrogates the regulation of receptor function by FKBP52 (52-kDa FK506-binding protein), which acts at a later stage of the chaperone cycle. Taken together, our data suggest a role for SGTA at distinct steps in the chaperone-dependent modulation of androgen, glucocorticoid, and progesterone receptor activity.

A cytosolic chaperone complexes with dynamic membrane J-proteins and mobilizes a nonenveloped virus out of the endoplasmic reticulum

Nonenveloped viruses undergo conformational changes that enable them to bind to, disrupt, and penetrate a biological membrane leading to successful infection. We assessed whether cytosolic factors play any role in the endoplasmic reticulum (ER) membrane penetration of the nonenveloped SV40. We find the cytosolic SGTA-Hsc70 complex interacts with the ER transmembrane J-proteins DnaJB14 (B14) and DnaJB12 (B12), two cellular factors previously implicated in SV40 infection. SGTA binds directly to SV40 and completes ER membrane penetration. During ER-to-cytosol transport of SV40, SGTA disengages from B14 and B12. Concomitant with this, SV40 triggers B14 and B12 to reorganize into discrete foci within the ER membrane. B14 must retain its ability to form foci and interact with SGTA-Hsc70 to promote SV40 infection. Our results identify a novel role for a cytosolic chaperone in the membrane penetration of a nonenveloped virus and raise the possibility that the SV40-induced foci represent cytosol entry sites.

Perilous journey: a tour of the ubiquitin-proteasome system

Eukaryotic cells are equipped to degrade proteins via the ubiquitin-proteasome system (UPS). Proteins become degraded upon their conjugation to chains of ubiquitin where they are then directed to the 26S proteasome, a macromolecular protease. The transfer of ubiquitin to proteins and their subsequent degradation are highly complex processes, and new research is beginning to uncover the molecular details of how ubiquitination and degradation take place in the cell. We review some of the new data providing insights into how these processes occur. Although distinct mechanisms are often observed, some common themes are emerging for how the UPS guides protein substrates through their final journey.

USP13 antagonizes gp78 to maintain functionality of a chaperone in ER-associated degradation

Physiological adaptation to proteotoxic stress in the endoplasmic reticulum (ER) requires retrotranslocation of misfolded proteins into the cytoplasm for ubiquitination and elimination by ER-associated degradation (ERAD). A surprising paradox emerging from recent studies is that ubiquitin ligases (E3s) and deubiquitinases (DUBs), enzymes with opposing activities, can both promote ERAD. Here we demonstrate that the ERAD E3 gp78 can ubiquitinate not only ERAD substrates, but also the machinery protein Ubl4A, a key component of the Bag6 chaperone complex. Remarkably, instead of targeting Ubl4A for degradation, polyubiquitination is associated with irreversible proteolytic processing and inactivation of Bag6. Importantly, we identify USP13 as a gp78-associated DUB that eliminates ubiquitin conjugates from Ubl4A to maintain the functionality of Bag6. Our study reveals an unexpected paradigm in which a DUB prevents undesired ubiquitination to sharpen substrate specificity for an associated ubiquitin ligase partner and to promote ER quality control. DOI: http://dx.doi.org/10.7554/eLife.01369.001.

Expression and clinical role of small glutamine-rich tetratricopeptide repeat (TPR)-containing protein alpha (SGTA) as a novel cell cycle protein in NSCLC

PURPOSE

A small glutamine-rich tetratricopeptide repeat-containing protein alpha (SGTA) is a 35 kDa protein involved in a number of biological processes. However, the role of SGTA in non-small-cell lung cancer (NSCLC) tumorigenesis has never been elucidated. The purpose of this study was to determine whether SGTA could serve as a biomarker for stratification and prediction of prognosis in NSCLC.

METHODS

Small glutamine-rich tetratricopeptide repeat-containing protein alpha expression was evaluated by Western blot in 8 paired fresh lung cancer tissues and immunohistochemistry on 83 paraffin-embedded sections. The effect of SGTA was assessed by RNA interference in A549 cells. Serum starvation and refeeding, flow cytometry, CCK-8, and tunnel assays were performed.

RESULTS

Small glutamine-rich tetratricopeptide repeat-containing protein alpha was highly expressed in NSCLC and significantly correlated with NSCLC histological differentiation, clinical stage, and Ki-67. Multivariate analysis indicated that SGTA was an independent prognostic factor for NSCLC patients' survival. The present investigation demonstrated that suppression of SGTA expression resulted in a significant decline of proliferation in A549 cells. Besides, SGTA could abolish the toxicity of cisplatin in A549 cells.

CONCLUSIONS

These findings suggested that SGTA might play an important role in promoting the tumorigenesis of NSCLC, and thus be a promising therapeutic target to prevent NSCLC progression.

A ubiquitin-like domain recruits an oligomeric chaperone to a retrotranslocation complex in endoplasmic reticulum-associated degradation

The Bag6-Ubl4A-Trc35 complex is a multifunctional chaperone that regulates various cellular processes. The diverse functions of Bag6 are supported by its ubiquitous localization to the cytoplasm, the nucleus, and membranes of the endoplasmic reticulum (ER) in cells. In ER-associated degradation (ERAD) pathways, Bag6 can interact with the membrane-associated ubiquitin ligase gp78 via its ubiquitin-like (UBL) domain, but the relative low affinity of this interaction does not reconcile with the fact that a fraction of Bag6 is tightly bound to the membranes. Here, we demonstrate that the UBL domain of Bag6 is required for interaction with the ER membranes. We find that in addition to gp78, the Bag6 UBL domain also binds a UBL-binding motif in UbxD8, an essential component of the gp78 ubiquitinating machinery. Importantly, Bag6 contains a proline-rich (PR) domain termed PDP (Proline rich-DUF3587-Proline rich) that forms homo-oligomer, allowing the UBL domain to form multivalent interactions with gp78 and UbxD8, which are essential for recruitment of Bag6 to the ER membrane. Furthermore, the PR domain comprises largely intrinsically disordered segments, which are sufficient for interaction with an unfolded substrate. We propose that simultaneous association with multiple ERAD factors helps to anchor a disordered chaperone oligomer to the site of retrotranslocation to prevent protein aggregation in ERAD.

All roads lead to Rome (but some may be harder to travel): SRP-independent translocation into the endoplasmic reticulum

Translocation into the endoplasmic reticulum (ER) is the first biogenesis step for hundreds of eukaryotic secretome proteins. Over the past 30 years, groundbreaking biochemical, structural and genetic studies have delineated one conserved pathway that enables ER translocation- the signal recognition particle (SRP) pathway. However, it is clear that this is not the only pathway which can mediate ER targeting and insertion. In fact, over the past decade, several SRP-independent pathways have been uncovered, which recognize proteins that cannot engage the SRP and ensure their subsequent translocation into the ER. These SRP-independent pathways face the same challenges that the SRP pathway overcomes: chaperoning the preinserted protein while in the cytosol, targeting it rapidly to the ER surface and generating vectorial movement that inserts the protein into the ER. This review strives to summarize the various mechanisms and machineries which mediate these stages of SRP-independent translocation, as well as examine why SRP-independent translocation is utilized by the cell. This emerging understanding of the various pathways utilized by secretory proteins to insert into the ER draws light to the complexity of the translocational task, and underlines that insertion into the ER might be more varied and tailored than previously appreciated.

The association of BAG6 with SGTA and tail-anchored proteins

Background

The BAG6 protein is a subunit of a heterotrimeric complex that binds a range of membrane and secretory protein precursors localized to the cytosol, enforcing quality control and influencing their subsequent fate.

Methodology and Principal Findings

BAG6 has an N-terminal ubiquitin-like domain, and a C-terminal Bcl-2-associated athanogene domain, separated by a large central proline-rich region. We have used in vitro binding approaches to identify regions of BAG6 important for its interactions with: i) the small-glutamine rich tetratricopeptide repeat-containing protein alpha (SGTA) and ii) two model tail-anchored membrane proteins as a paradigm for its hydrophobic substrates. We show that the BAG6-UBL is essential for binding to SGTA, and find that the UBL of a second subunit of the BAG6-complex, ubiquitin-like protein 4A (UBL4A), competes for SGTA binding. Our data show that this binding is selective, and suggest that SGTA can bind either BAG6, or UBL4A, but not both at the same time. We adapted our in vitro binding assay to study the association of BAG6 with an immobilized tail-anchored protein, Sec61β, and find both the UBL and BAG domains are dispensable for binding this substrate. This conclusion was further supported using a heterologous subcellular localization assay in yeast, where the BAG6-dependent nuclear relocalization of a second tail-anchored protein, GFP-Sed5, also required neither the UBL, nor the BAG domain of BAG6.

Significance

On the basis of these findings, we propose a working model where the large central region of the BAG6 protein provides a binding site for a diverse group of substrates, many of which expose a hydrophobic stretch of polypeptide. This arrangement would enable the BAG6 complex to bring together its substrates with potential effectors including those recruited via its N-terminal UBL. Such effectors may include SGTA, and the resulting assemblies influence the subsequent fate of the hydrophobic BAG6 substrates.

Bag6/Bat3/Scythe: a novel chaperone activity with diverse regulatory functions in protein biogenesis and degradation

Upon emerging from the ribosome exiting tunnel, polypeptide folding occurs immediately with the assistance of both ribosome-associated and free chaperones. While many chaperones known to date are dedicated folding catalysts, recent studies have revealed a novel chaperoning system that functions at the interface of protein biogenesis and quality control by using a special "holdase" activity in order to sort and channel client proteins to distinct destinations. The key component, Bag6/Bat3/Scythe, can effectively shield long hydrophobic segments exposed on the surface of a polypeptide, preventing aggregation or inappropriate interactions before a triaging decision is made. The biological consequences of Bag6-mediated chaperoning are divergent for different substrates, ranging from membrane integration to proteasome targeting and destruction. Accordingly, Bag6 can act in various cellular contexts in order to execute many essential cellular functions, while dysfunctions in the Bag6 system can cause severe cellular abnormalities that may be associated with some pathological conditions.