Binding of SGTA to Rpn13 selectively modulates protein quality control

Leznicki, Pawel; Korac-Prlic, Jelena; Kliza, Katarzyna; Husnjak, Koraljka; Nyathi, Yvonne; Dikic, Ivan; High, Stephen

Our previous studies indicated that SGTA antagonises the BAG6-dependent ubiquitylation and proteasomal degradation of MLPs, a role that is particularly apparent when cellular SGTA levels are increased following its overexpression (Leznicki and High, 2012; Wunderley et al., 2014). We now offer a molecular basis for this effect by providing evidence that SGTA can regulate the access of MLPs to the proteasomal core in a manner that leaves other substrates, as exemplified by Ub-R-GFP, unaffected. Thus, the increased MLP levels observed upon SGTA overexpression correlate with an increase in the association of the MLP, and both exogenous and endogenous SGTA, with the proteasome. Similar to the effect of an SGTA knockdown (Wunderley et al., 2014), co-expression of an Rpn13 C-terminal region reduces steady-state MLP levels, and our data are consistent with a partial displacement of endogenous SGTA from the proteasome. This effect of Rpn13 is even more striking in the context of SGTA overexpression, which normally stabilises MLPs leading to their non-physiological accumulation (Wunderley et al., 2014). Under these circumstances, Rpn13150-407 co-expression substantially reduces the binding of exogenous SGTA to the proteasome and reverses the SGTA mediated increase in steady-state MLP levels. These findings are consistent with a model in which Rpn13-bound SGTA binds to MLPs and delays their proteasomal degradation (see Fig. 7; cf. Wunderley et al., 2014).