DIFFERENTIAL REGULATION OF HSP70 SUBFAMILIES BY THE EUKARYOTIC DNAJ HOMOLOG YDJ1

In Saccharomyces cerevisiae Ydj1p, a DnaJ homolog, is localized to the cytosol with the Ssa and Ssb Hsp70 proteins. Ydj1p helps facilitate p olypeptide translocation across mitochondrial and endoplasmic reticulu m membranes (Caplan, A. J., Cyr, D. M., and Douglas, M. G. (1992) Cell 71, 1143-1155) and can directly interact with Ssa1p to regulate chape rone activity (Cyr, D. M., Lu, X., and Douglas, M. G. (1992) J. Biol. Chem. 267, 20927-20931). In this study, the role of Ydj1p in modulatin g ATP-dependent reactions catalyzed by Ssa and Ssb Hsp70 proteins has been examined using purified components and compared with that of othe r Hsp70 homologs BiP and DnaK. Ssa1p, Ssa2p, and Ssb1/2p all formed st able complexes with the mitochondrial presequence peptide, F1beta(1-51 ). ATP alone had only modest effects on polypeptide complex formation with Ssa1p and Ssa2p, but prevented the majority of polypeptide bindin g to BiP and DnaK. ATP by itself also reduced polypeptide binding to S sb1/2p to a level that was intermediate between that observed for the Ssa Hsp70 proteins tested and BiP and DnaK. ATP hydrolysis by Ssa1p, S sa2p, and Ssb1/2p occurred at similar rates. Ydj1p was a potent modula tor of the both the ATPase and polypeptide binding activities of Ssa1p and Ssa2p. In contrast, Ydj1p had little effect on the ATPase and pol ypeptide binding activity of Ssb1/2p. Therefore the chaperone-related activities of Ssa and Ssb Hsp70 proteins exhibit significant differenc es in sensitivity to ATP and YDJ1p. These data indicate that regulatio n of Hsp70 activity by DnaJ homologs can be specific. The specificity of interactions between Ydj1p and the Ssa and Ssb Hsp70 proteins obser ved could contribute in determining the functional specificity of thes e chaperones in the cytosol. In related experiments, F1beta(1-51) was found to reduce the extent to which Ydj1p stimulated Ssa1p ATPase acti vity. This effect correlated with the formation of F1beta(1-51)-Ssa1p complexes. We propose that intramolecular communication between the po lypeptide binding, ATPase and DnaJ regulatory domains on Ssa1p plays a role in the regulation of chaperone activity.