MUTATION OF AN EF-HAND CA2+-BINDING MOTIF IN PHOSPHOLIPASE-C OF DICTYOSTELIUM-DISCOIDEUM - INHIBITION OF ACTIVITY BUT NO EFFECT ON CA2+-DEPENDENCE

Phosphoinositide-specific phospholipase C (PLC) is dependent on Ca2+ ions for substrate hydrolysis. The role of an EF-hand Ca2+-binding motif in Ca2+-dependent PLC activity was investigated by site-directed mutagenesis of the Dictyostelium discoideum PLC enzyme. Amino acid residues with oxygen-containing side chains at co-ordinates x, y, z,-x and -z of the putative Ca2+-binding-loop sequence were replaced by isoleucine (x), valine (y) or alanine (z, -x and -z). The mutated proteins were expressed in a Dictyostelium cell line with a disrupted pie gene displaying no endogenous PLC activity, and PLC activity was measured in cell lysates at different Ca2+ concentrations. Replacement of aspartate at position x, which is considered to play an essential role in Ca2+ binding, had little effect on Ca2+ affinity and maximal enzyme activity. A mutant with substitutions at both aspartate residues in position x and y also showed no decrease in Ca2+ affinity, whereas the maximal PLC activity was reduced by 60%. Introduction of additional mutations in the EF-hand revealed that the Ca2+ concentration giving half-maximal activity was unaltered, but PLC activity levels at saturating Ca2+ concentrations were markedly decreased. The results demonstrate that, although the EF-hand domain is required for enzyme activity, it is not the site that regulates the Ca2+-dependence of the PLC reaction.