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.