THE 46/50-KDA PHOSPHOPROTEIN VASP PURIFIED FROM HUMAN PLATELETS IS A NOVEL PROTEIN ASSOCIATED WITH ACTIN-FILAMENTS AND FOCAL CONTACTS

Vasoactive agents which elevate either cGMP or cAMP inhibit platelet activation by pathways sharing at least one component, the 46/50 kDa vasodilator-stimulated phosphoprotein (VASP). VASP is stoichiometrically phosphorylated by both cGMP-dependent and cAMP-dependent protein kinases in intact human platelets, and its phosphorylation correlates very well with platelet inhibition caused by cGMP- and cAMP-elevating agents. Here we report that in human platelets spread on glass, VASP is associated predominantly with the distal parts of radial microfilament bundles and with microfilaments outlining the periphery, whereas less VASP is associated with a central microfilamentous ring. VASP is also detectable in a variety of different cell types including fibroblasts and epithelial cells. In fibroblasts, VASP is concentrated at focal contact areas, along microfilament bundles (stress fibres) in a punctate pattern, in the periphery of protruding lamellae, and is phosphorylated by cGMP- and cAMP-dependent protein kinases in response to appropriate stimuli. Evidence for the direct binding of VASP to F-actin is also presented. The data demonstrate that VASP is a novel phosphoprotein associated with actin filaments and focal contact areas, i.e. transmembrane junctions between microfilaments and the extracellular matrix.