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

In seeking so-far-unknown functions for the intrinsic proteasomal ubiquitin receptor Rpn13, we identified a novel interaction with SGTA. This interaction was initially suggested by the results of a yeast two-hybrid screen and, subsequently, validated using two different pull-down strategies, which showed that the C-terminal region of Rpn13 binds to the central tetratricopeptide repeat (TPR) domain of SGTA. Since Rpn13 acts as a proteasomal ubiquitin receptor, we speculated that SGTA influences substrate access to the proteasome. To test this hypothesis, we investigated the potential role of the Rpn13–SGTA interaction in the proteasomal degradation of MLPs, a process previously shown to be regulated by both SGTA (Leznicki and High, 2012; Wunderley et al., 2014) and its interacting partner BAG6 (Hessa et al., 2011; Rodrigo-Brenni et al., 2014). We show that the binding of exogenous SGTA to Rpn13 results in a substantial increase in the steady-state level of MLPs. Inhibiting this interaction by overexpression of the Rpn13 C-terminal region or mutation of the SGTA TPR region negates the effect of SGTA overexpression on MLP levels. These data support a model whereby SGTA and its interacting partner the BAG6 complex can influence the fate of MLPs at the proteasome. We speculate that these components modulate the access of such substrates to the proteasome through their respective partners Rpn13 (this study) and Rpn10 (Kikukawa et al., 2005; Minami et al., 2010). This model suggests that SGTA and BAG6 control the fate of MLPs even after their arrival at the proteasome, and provides the basis for a potential substrate rescue pathway and/or a mechanism to enhance the selectivity of substrate degradation.