Protein targeting and degradation are coupled for elimination of mislocalized proteins

Abstract

A substantial proportion of the genome encodes membrane proteins that are delivered to the endoplasmic reticulum by dedicated targeting pathways1. Membrane proteins that fail targeting must be rapidly degraded to avoid aggregation and disruption of cytosolic protein homeostasis2,3. The mechanisms of mislocalized protein (MLP) degradation are unknown. Here, we reconstitute MLP degradation in vitro to identify factors involved in this pathway. We find that nascent membrane proteins tethered to ribosomes are not substrates for ubiquitination unless they are released into the cytosol. Their inappropriate release results in capture by the Bag6 complex, a recently identified ribosome-associating chaperone4. Bag6 complex capture depends on unprocessed or non-inserted hydrophobic domains that distinguish MLPs from potential cytosolic proteins. A subset of these Bag6 clients is transferred to TRC40 for membrane insertion, while the remainder are rapidly ubiquitinated. Depletion of the Bag6 complex impairs efficient ubiquitination selectively of MLPs. Thus, by its presence on ribosomes synthesizing nascent membrane proteins, the Bag6 complex links targeting and ubiquitination pathways. We propose that such coupling permits fast-tracking of MLPs for degradation without futile engagement of cytosolic folding machinery.

Chaperone BAG6 is dispensable for MHC class I antigen processing and presentation

“…While assisting in protein biosynthesis on the one hand, BAG6 also mediates degradation of mislocalized nascent chains (Hessa et al, 2011) and ERAD substrates (Claessen and Ploegh, 2011;Claessen et al, 2014;Payapilly and High, 2014;Wang et al, 2011). Moreover, Rodrigo-Brenni et al (2014) recently described RNF126 as an E3 ligase interacting with BAG6 and specifically ubiquitylating chaperone bound client proteins (Rodrigo-Brenni et al, 2014).…”

Involvement of Bag6 and the TRC pathway in proteasome assembly

“…Bag6 is a ribosome-associating chaperone that recognizes hydrophobic domains of nascent proteins and regulates protein triage, in which proteins undergo either productive folding and membrane targeting or ubiquitination and degradation 38,42,43 . Our findings add another layer of regulation for protein homoeostasis by Bag6 and the TRC pathway and have revealed that proteasome assembly is not an isolated system that is catalysed solely by dedicated chaperones but also requires general chaperones that preserve protein homoeostasis.…”

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

“…[1][2][3]. Recent studies demonstrated that Bag6 forms a heterotrimeric Bag6 complex with ubiquitin-like 4A (Ubl4A) and transmembrane domain recognition complex 35 (TRC35) (4,5) that mediates the fates of membrane proteins in tail-anchor (TA) protein targeting (6), mislocalized protein degradation (7), and endoplasmic reticulum (ER)-associated protein degradation (4). The many roles of the Bag6 complex likely are centered on its ability to bind exposed hydrophobic regions of proteins, such as transmembrane domains.…”