How potassium affects the activity of the molecular chaperone Hsc70. I. Potassium is required for optimal ATPase activity.
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 approximately 0.1 M [K+]. Activation of the ATPase turnover correlates with the ionic radii of monovalent ions; those that are at least 0.3 A smaller (Na+ and Li+) or larger (Cs+) than K+ show negligible activation, whereas ions with radii differing only approximately 0.1 A 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.
O'Brien
M C
MC
Beckman Laboratories for Structural Biology, Stanford University School of Medicine, California 94305.
McKay
D B
DB
D000244:Adenosine Diphosphate / Q000378:metabolism; D000251:Adenosine Triphosphatases / Q000378:metabolism*; D000255:Adenosine Triphosphate / Q000378:metabolism; D000818:Animals; D002352:Carrier Proteins / Q000502:physiology*; D002414:Cations, Monovalent; D002417:Cattle; D050883:HSC70 Heat-Shock Proteins; D018840:HSP70 Heat-Shock Proteins; D066298:In Vitro Techniques; D007700:Kinetics; D018832:Molecular Chaperones; D011188:Potassium / Q000502:physiology*; D011994:Recombinant Proteins
D002352:Carrier Proteins; D002414:Cations, Monovalent; D050883:HSC70 Heat-Shock Proteins; D018840:HSP70 Heat-Shock Proteins; D018832:Molecular Chaperones; D011994:Recombinant Proteins; D000244:Adenosine Diphosphate; D000255:Adenosine Triphosphate; D000251:Adenosine Triphosphatases; D011188:Potassium