In a previous study, we showed that the molecular chaperone HSC70 self-associates in solution in a reversible and likely unlimited fashion. Here, we examine the influence of nucleotides, nucleotide analogs, peptides, and unfolded proteins on the self-association properties of this protein. Whereas in the presence of ADP, HSC70 exists as a slow, concentration-and temperaturedependent monomer-oligomer equilibrium, in the presence of ATP, the protein is essentially monomeric, indicating that ATP shifts this equilibrium toward the monomer by stabilizing the monomer. Dissociation of oligomers into monomers is also obtained with the slowly hydrolyzable ATP analogs, adenosine 5-O-(thiotriphosphate) and 5-adenylyl-␤,␥-imidodiphosphate, or the complex between ADP and the phosphate analog, BeF 3 , indicating that binding but not hydrolysis of ATP is necessary and sufficient for the stabilization of HSC70 monomer. Furthermore, binding of short peptides or permanently unfolded proteins to the peptide binding site of HSC70 promotes the dissociation of oligomers into monomers, suggesting that protein substrates are able to compete with HSC70 for the same binding site. Because the release of peptides or unfolded proteins from HSC70 has also been shown to require ATP binding, these results indicate that dissociation of oligomers is controlled by a mechanism similar to that of release of protein substrates and suggest that binding of HSC70 to itself occurs via the peptide binding site and mimics binding of HSC70 to protein substrates.The correct folding of proteins is thought to be the result of a kinetic competition between a productive "on-pathway" leading to the native, biologically active, state and an abortive "offpathway" leading to aggregation of incompletely folded intermediates (1-4). In vivo, the efficiency of the folding process depends on the presence of cellular factors such as molecular chaperones, which by transiently binding to unfolded or partially folded polypeptide chains prevent incorrect intra-and intermolecular interactions, thus favoring the productive pathway over aggregation (4 -6). Among these factors, the 70-kDa heat shock cognate protein (HSC70), a constitutively expressed member of the highly conserved 70-kDa heat shock protein (HSP70) family, plays an essential role not only in protein folding but also in protein biosynthesis, assembly, transport, degradation, and signal transduction (for reviews see Refs. 6 -9).HSC70, purified originally as an ATPase that uncoats clathrin-coated vesicles (10 -12), has a weak intrinsic ATPase activity (13, 14) but binds tightly to ATP and ADP (15-18). Upon binding to clathrin light chains, synthetic peptides, unfolded proteins, or other heat shock proteins such as DnaJ homologs, HSC70 ATPase activity is stimulated 2-5-fold (13, 16, 19 -28).Limited proteolysis and mutagenesis studies have shown that the protein is made of two domains, a 44-kDa aminoterminal ATPase domain that binds and hydrolyzes ATP and a 30-kDa carboxyl-terminal domain involved in the binding of th...