HSP70-binding motifs function as protein quality control degrons

Abstract

AbstractProtein quality control (PQC) degrons are short protein segments that target misfolded proteins for degradation through the ubiquitin-proteasome system (UPS). To uncover how PQC degrons function, we performed a screen in Saccharomyces cerevisiae by fusing a library of flexible tetrapeptides to the C-terminus of the Ura3-HA-GFP reporter. The identified degrons exhibited high sequence variation but with marked hydrophobicity. Notably, the best scoring degrons constitute predicted Hsp70-binding motifs. When directly tested, a canonical Hsp70 binding motif (RLLL) functioned as a dose-dependent PQC degron that was targeted by Hsp70, Hsp110, Fes1, several Hsp40 J-domain co-chaperones and the PQC E3 ligase Ubr1. Our results suggest that multiple PQC degrons overlap with chaperone-binding sites and that PQC-linked degradation achieves specificity via chaperone binding. Thus, the PQC system has evolved to exploit the intrinsic capacity of chaperones to recognize misfolded proteins, thereby placing them at the nexus of protein folding and degradation.