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

We next asked whether a reduction of the proteasome-associated fraction of exogenous SGTA by overexpressing Rpn13150-407 has any impact on the fate of OP91 (see Fig. 3) and an alternative MLP OPG-TASK85 that is derived from the K+-channel TASK-1 (Wunderley et al., 2014). Co-expression of Rpn13150-407 and the MLPs alone led to a reduction in detectable amounts of OP91and OPG-TASK85 (Fig. 5A and B, lanes 1 and 4, see also accompanying graphs); suggesting that the Rpn13150-407-mediated displacement of endogenous SGTA (Fig. 4Bii) reduces steady-state levels of MLP. The effect of Rpn13150-407 co-expression is much more striking when exogenous SGTA is present, with steady-state levels of MLP approaching those seen without exogenous SGTA (Fig. 5A and B, cf. lanes 1, 5 and 8; see also accompanying graphs). Co-expressing full-length Rpn13 with exogenous SGTA had an effect that is comparable with that of Rpn13150-407 for the MLP substrate OPG-TASK85, but is more-modest in the case of OP91 (Fig. 5A and B, lanes 5–8). By contrast, co-expression of the N-terminal Rpn131-150 domain in combination with SGTA has relatively little effect (Fig. 5A and B, cf. lanes 5, 7 and 8). The substrate specificity of these effects was explored by using ubiquitin–arginine–GFP (Ub-R-GFP) (Dantuma et al., 2000), a proteasomal N-end rule substrate that appears to be insensitive to changes in SGTA levels (Leznicki and High, 2012; Wunderley et al., 2014). Despite comparable levels of expression of SGTA and all three Rpn13 variants (Fig. 5A–C), steady-state levels of Ub-R-GFP were essentially unaltered by any of the combinations tested (Fig. 5C, lanes 1–8; and accompanying graph). On the basis of these results, we conclude that the ability of exogenous SGTA to enhance steady-state MLP levels is strongly dependent upon its binding to the C-terminal region of Rpn13.