REGULATION OF 70-KDA HEAT-SHOCK-PROTEIN ATPASE ACTIVITY AND SUBSTRATE-BINDING BY HUMAN DNAJ-LIKE PROTEINS, HSJ1A AND HSJ1B

The DnaJ family of molecular chaperones is characterized by the presen ce of a highly conserved 70-amino-acid J domain. Escherichia coli DnaJ interacts with the 70-kDa heat-shock protein (DnaK), in vitro, to sti mulate the 70-kDa heat-shock protein ATPase activity and modify substr ate binding. The conservation of the interaction of DnaJ-like proteins with the 70-kDa heat-shock proteins has been demonstrated for the yea st protein YDJ1, a protein that shows full domain conservation with E. coli DnaJ. Human neurone-specific Dual-like proteins, HSJ1a and HSJ1b , possess a J domain and a glycine/phenylalanine-rich region in common with E. coli DnaJ, although the overall amino acid identity is less t han 23%. We have investigated, in vitro, the interaction of HSJ1a and HSJ1b with the mammalian brain constitutive 70-kDa heat-shock protein (hsc70). The weak intrinsic ATPase activity of the constitutive 70-kDa heat-shock protein is enhanced more than fivefold by stoichiometric a mounts of both HSJ1a and HSJ1b. This enhancement is mediated by an inc rease in the rate of bound ATP hydrolysis, whereas the rate of ADP rel ease is unaffected. HSJ1 proteins appear to regulate the affinity of t he 70-kDa constitutive heat-shock protein for the permanently unfolded substrate, carboxymethylated alpha-lactalbumin. A recent report [Pall eros, D. R., Reid, K. L., Shi, L., Welch, W. J. & Fink, A. L. (1993) N ature 365, 664-666] has suggested that substrate release by 70-kDa hea t-shock proteins requires a conformational change in these proteins in duced by K+ in concert with ATP binding. In the presence of ATP, HSJ1 proteins reduce 70-kDa constitutive heat-shock protein/carboxymethylat ed alpha-lactalbumin complex formation both in the presence and absenc e of Kf, This suggests that HSJ1 proteins induce a conformational chan ge in the 70-kDa constitutive heat-shock protein that can mimic the ef fect mediated by K+ and therefore modulate 70-kDa heat-shock protein s ubstrate release by another mechanism rather than merely stimulating t he 70-kDa heat-shock protein ATPase activity. As HSJ1 proteins have li mited similarity to DnaJ, we suggest that this action is being mediate d by the J domain alone, and that this modulation of 70-kDa heat-shock -protein substrate binding will be common to all proteins that contain a J domain.