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Specificity of the interaction between phosphatidylinositol 4,5‐bisphosphate and the profilin:actin complex

Profilactin, the profiling:actin complex, which is present in large amounts in extracts of many types of eukaryotic cells, appears to serve as the precursor of microfilaments. It was reported recently that profilactin interacts specifically with phosphatidylinositol 4,5‐bisphosphate (PtdIns(4,5)P2) (Lassing and Lindberg: Nature 314:472–474, 1985.) The present paper describes in detail the behaviour of profilactin and profilin in the presence of different types of phospholipids and neutral lipids under different conditions. PtdIns(4,5)P2 is the only phospholipid found so far which in the presence of 80 mM KC1 and at Ca2+ concentrations below 10−5 M effectively dissociates profilactin with the resulting polymerization of the actin. Phosphatidylinositol 4‐monophosphate exhibits some activity but phosphatidylinositol is inactive. Both calf spleen profilin and profilin from human platelets form stable complexes with PtdIns(4,5)P2 micelles. PtdIns(4,5)P2 is active also when incorporated together with other phospholipids in mixed vesicles.