SGTA antagonizes BAG6-mediated protein triage

Pawel Leznicki, Stephen High

Research output: Contribution to journalArticlepeer-review

Abstract

The BAG6 complexwas first identified as an upstreamloading 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 α (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.
Original languageEnglish
Pages (from-to)19214-19219
Number of pages5
JournalProceedings of the National Academy of Sciences of the United States of America
Volume109
Issue number47
DOIs
Publication statusPublished - 20 Nov 2012

Keywords

  • Membrane protein targeting
  • Protein homeostasis

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