Modulation of the GTPase activity of the chloroplast F1-ATPase by ATP binding at noncatalytic sites.

Although the binding of nucleotides at the noncatalytic sites of F1-ATPase has been regarded as probably having some type of regulatory function, only limited observations have been reported that support such a role. We present here results showing that the presence of ATP at noncatalytic sites can give a fivefold enhancement of the rate of GTP hydrolysis by the chloroplast F1-ATPase. Heat-activation of the chloroplast F1-ATPase in the presence of ATP, followed by column separation from the medium nucleotides gives an enzyme with two of the three noncatalytic sites filled with ATP. In contrast, heat-activation in the presence of ADP gives an enzyme with only one noncatalytic site filled with ADP. Such an enzyme with two noncatalytic sites empty catalyzes MgGTP hydrolysis only very slowly. The filling of a second noncatalytic site with ATP by exposure of the enzyme to ATP without Mg2+ present, followed by column separation, markedly increases the rate of GTP hydrolysis. A further increase occurs when a third noncatalytic site is filled by exposure to Mg2+ and ATP. The rate of MgATP hydrolysis is the same for the enzyme heat-activated in the presence of ATP or ADP, probably because MgATP, unlike MgGTP, rapidly binds to both catalytic and noncatalytic sites.