Association of α-phosphatidylinositol-specific phospholipase C with phospholipid vesicles

The alpha isoform of phosphatidylinositol-specific phospholipase C (α-PI-PLC, M_r 62 000) was purified from bovine brain. Enzyme activity was dependent on calcium, sodium cholate and showed the anticipated specificity for the phosphatidylinositols. Calcium interaction with this protein, investigated by gel filtration chromatography, showed no detectable binding at calcium concentrations adequate to activate the enzyme. Association of α-PI-PLC with phospholipid vesicles was studied by light scattering, fluorescence energy transfer and gel-filtration chromatography. The enzyme readily associated with vesicles of high charge density, with vesicles of crude acidic phospholipids and with PIP₂. Interaction was characterized by a rapid association followed by slower addition of more protein to the phospholipid. Complexes containing 20-30 percent protein (by weight) were readily obtained. Calcium had only a small effect on this interaction. The protein-phospholipid complexes appeared to bind less calcium than a similar amount of phospholipid alone. Thus, α-PI-PLC did not appear to be a calcium-binding protein in either its free or membrane-associated states. Although α-PI-PLC showed the highest propensity to bind to phospholipids, a number of other proteins also associated with phospholipids under the conditions used. Thus, whether or not the observed interaction of α-PI-PLC with membranes was specific and biologically important or whether it was a process common to many proteins, was not known. Knowledge of this interaction may enhance our understanding of possible mechanisms for protein-membrane interactions in general.