ALPHA-ACTININ AND VINCULIN ARE PIP(2)-BINDING PROTEINS INVOLVED IN SIGNALING BY TYROSINE KINASE

Western blot analysis of Balb/c 3T3 cell lysates by an antibody specif ic to phosphatidylinositol 4,5-bisphosphate (PIP2) showed that several proteins exist in a PIP2-bound form. Among them, two proteins, 100 an d 115 kDa in molecular mass, were detected as PIP2 abundant proteins. These were identified as alpha-actinin and vinculin by their antibodie s. In Balb/c 3T3 cells, alpha-actinin in the cytoskeleton contains PIP 2, while alpha-actinin in cytosol does not. The levels of PIP2 bound t o alpha-aetinin decrease in response to platelet-derived growth factor (PDGF). Similarly, PIP2 bound to vinculin is decreased upon stimulati on with PDGF. By immunofluorescent staining, PIP2 was found to be pres ent densely in the central areas around nuclei, microfilament bundles, and focal contacts, where alpha-actinin and vinculin are distributed. PDGF stimulation decreases the intensity of PIP2 staining in these ar eas. In this paper we suggest that tyrosine kinase-activated phospholi pase C hydrolyzes PIP2 bound to alpha-actinin and vinculin, leading to the simultaneous generation of second messengers and reorganization o f the cytoskeleton.