Rsp5/Nedd4 is the major ubiquitin ligase that targets cytosolic misfolded proteins upon heat-stress

The heat-shock response is a complex cellular program that induces major changes in protein translation, folding and degradation to alleviate toxicity caused by protein misfolding. While heat-shock has been widely used to study proteostasis, it remained unclear how misfolded proteins are targeted for proteolysis in these conditions. We found that Rsp5 and its mammalian homologue Nedd4 are ones of the main E3-ligases responsible for the increased ubiquitination induced by heat-stress. We determined that Rsp5 ubiquitinates mainly cytosolic misfolded proteins upon heat-shock for proteasome degradation. We found that ubiquitination of heat-induced substrates requires the Hsp40 co-chaperone Ydj1 that is further associated with Rsp5 upon heat-shock. Additionally, ubiquitination is also promoted by PY Rsp5-binding motifs found primarily in the structured regions of stress-induced substrates, which can act as heat-induced degrons. Our results support a bipartite recognition mechanism combining direct and chaperone-dependent ubiquitination of misfolded cytosolic proteins by Rsp5.