An essential role for the substrate-binding region of hsp40s in Saccharomyces cerevisiae

In addition to regulating the ATPase cycle of Hsp70, a second critical role of Hsp40s has been proposed based on in vitro studies: binding to denature d protein substrates, followed by their presentation to Hsp70 for folding. However, the biological importance of this model is challenged by the fact that deletion of the substrate-binding domain of either of the two major Hs p40s of the yeast cytosol, Ydj1 and Sis1, leads to no severe defects, as lo ng as regions necessary for Hsp70 interaction are retained. As an in vivo t est of this model, requirements for viability were examined in a strain hav ing deletions of both Hsp40 genes. Despite limited sequence similarity, the substrate-binding domain of either Sis1 or Ydj1 allowed cell growth, indic ating they share overlapping essential functions. Furthermore, the substrat e-binding domain must function in cis with a functional Hsp70-interacting d omain. We conclude that the ability of cytosolic Hsp40s to bind unfolded pr otein substrates is an essential function in vivo.