HOW POTASSIUM AFFECTS THE ACTIVITY OF THE MOLECULAR CHAPERONE HSC70 .1. POTASSIUM IS REQUIRED FOR OPTIMAL ATPASE ACTIVITY

Several functions of the 70-kilodalton heat shock cognate protein (Hsc 70), such as peptide binding/release and clathrin uncoating, have been shown to require potassium ions. We have examined the effect of monov alent ions on the ATPase activity of Hsc70. The steady-state ATPase ac tivities of Hsc70 and its amino-terminal 44-kDa ATPase fragment are mi nimal in the absence of K+ and reach a maximum at similar to 0.1 M [K]. Activation of the ATPase turnover correlates with the ionic radii o f monovalent ions; those that are at least 0.3 Angstrom smaller (Na+ a nd Li+) or larger (Cs+) than K+ show negligible activation, whereas io ns with radii differing only similar to 0.1 Angstrom from that of K+ ( NH4+ and Rb+) activate to approximately half the turnover rate observe d with K+. Single turnover experiments with Hsc70 demonstrate that ATP hydrolysis is 5-fold slower with Na+ than with K+. The equilibrium bi nding of ADP or ATP to Hsc70 is unperturbed when K+ is replaced with N a+. These results are consistent with a role for monovalent ions as sp ecific cofactors in the enzymatic hydrolysis of ATP.