Activation of pleiotropic drug resistance by the J-protein and Hsp70-related proteins, Zuo1 and Ssz1

Ssz1 (Pdr13) and Zuo1, members of the Hsp70 and J-protein molecular chaperone families, respectively, form a heterodimer and function on the ribosome with the Hsp70, Ssb, presumably assisting folding of newly synthesized polypeptides. As it has also been reported that Ssz1 induces pleiotropic drug resistance (PDR) when overexpressed, a possible role for Zuo1 in PDR was investigated. The C-terminal domain of Zuo1, which is dispensable for Zuo1's chaperone function on the ribosome, is both necessary and sufficient for PDR induction by Zuo1. A single domain of Ssz1, the N-terminal ATPase domain, is sufficient for PDR induction as well, indicating that Ssz1 does not function as a chaperone in PDR. No role for Ssb was found in PDR; overexpression did not affect PDR, nor was its presence required for Ssz1's or Zuo1's effect on PDR. As our results also indicate that Ssz1 and Zuo1 must be free of ribosomes to induce PDR, we propose that Ssz1's and Zuo1's function in PDR is distinct from their role as ribosome-associated co-chaperones and may be regulatory in nature.