Hsp70-Hsp110 chaperones deliver ubiquitin-dependent and -independent substrates to the 26S proteasome for proteolysis in yeast.

During protein quality control, proteotoxic misfolded proteins are recognized by molecular chaperones, ubiquitylated by dedicated quality control ligases and delivered to the 26S proteasome for degradation. Proteins belonging to the Hsp70 chaperone and Hsp110 (the Hsp70 nucleotide exchange factor) families function in the degradation of misfolded proteins by the ubiquitin-proteasome system via poorly understood mechanisms. Here, we report that the Hsp110 proteins (Sse1 and Sse2) function in the degradation of Hsp70-associated ubiquitin conjugates at the post-ubiquitylation step and are also required for ubiquitin-independent proteasomal degradation. Hsp110 associates with the 19S regulatory particle of the 26S proteasome and interacts with Hsp70 to facilitate the delivery of Hsp70 substrates for proteasomal degradation. By using a highly defined ubiquitin-independent proteasome substrate, we show that the mere introduction of a single Hsp70-binding site renders its degradation dependent on Hsp110. The findings define a dedicated and chaperone-dependent pathway for the efficient shuttling of cellular proteins to the proteasome with profound implications for understanding protein quality control and cellular stress management.