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SGTA antagonizes BAG6-mediated protein triage.

ABSTRACT: The BAG6 complex was first identified as an upstream loading factor for tail-anchored membrane proteins entering the TRC40-dependent pathway for posttranslational delivery to the endoplasmic reticulum. Subsequently, BAG6 was shown to enhance the proteasomal degradation of mislocalized proteins by selectively promoting their ubiquitination. We now show that the BAG6-dependent ubiquitination of mislocalized proteins is completely reversible and identify a pivotal role for the small glutamine-rich tetratricopeptide repeat-containing protein ? (SGTA) in specifically antagonizing this process. SGTA does not simply mask the exposed hydrophobic transmembrane domain of a mislocalized protein, thereby preventing BAG6 recruitment. Rather, SGTA actively promotes the deubiquitination of mislocalized proteins that are already covalently modified, thus reversing the actions of BAG6 and inhibiting its capacity to promote substrate-specific degradation. This SGTA-mediated effect is independent of its tetratricopeptide motifs, suggesting it does not require the actions of Hsp70 and Hsp90 chaperones. These data reveal that, in a cellular context, mislocalized protein ubiquitination is the result of a dynamic equilibrium reflecting competition between pathways that promote either protein maturation or degradation. The targeted perturbation of this equilibrium, achieved by increasing steady-state SGTA levels, results in a specific stabilization of a model mislocalized protein derived from the amyloid precursor protein, an effect that is completely negated by ensuring efficient precursor delivery to the endoplasmic reticulum. We speculate that a BAG6/SGTA cycle operates during protein maturation and quality control in the cytosol and that together these components dictate the fate of a specific subset of newly synthesized proteins.

SUBMITTER: Leznicki P

PROVIDER: S-EPMC3511132 | biostudies-literature | 2012 Nov

REPOSITORIES: biostudies-literature

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SGTA antagonizes BAG6-mediated protein triage.

Leznicki Pawel P High Stephen S

Proceedings of the National Academy of Sciences of the United States of America 20121105 47

The BAG6 complex was first identified as an upstream loading factor for tail-anchored membrane proteins entering the TRC40-dependent pathway for posttranslational delivery to the endoplasmic reticulum. Subsequently, BAG6 was shown to enhance the proteasomal degradation of mislocalized proteins by selectively promoting their ubiquitination. We now show that the BAG6-dependent ubiquitination of mislocalized proteins is completely reversible and identify a pivotal role for the small glutamine-rich ...[more]

PMID: 23129660

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Project description:BackgroundThe 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 findingsBAG6 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.SignificanceOn 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.

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SGTA antagonizes BAG6-mediated protein triage.

Publications

SGTA antagonizes BAG6-mediated protein triage.

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