How Potassium Affects the Activity of the Molecular Chaperone Hsc70

Abstract

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