Phosphorylation of vinculin in human platelets spreading on a solid surface.

Vinculin is a cytoskeletal protein believed to be involved in linking microfilaments to the cell membrane. It is a substrate for the Ca(2+)- and phospholipid-dependent protein kinase C. We show here that when human platelets attach and spread on a solid surface, the alpha isoforms of vinculin become phosphorylated at serine and/or threonine residues. Phosphorylation is dependent on adhesion to a surface, since suspended, unattached platelets can produce filopodia but no phosphorylation of vinculin. Phosphorylation is also dependent on actin polymerization, as it does not occur when platelets had been pretreated with cytochalasin B. Most likely, protein kinase C is responsible for the phosphorylation of vinculin, since phosphorylation also occurs when platelets are treated with a phorbol ester, which activates protein kinase C, and is blocked by treatment with a staurosporine derivative which inhibits this enzyme. These results suggest that phosphorylation plays a role in anchoring vinculin at sites of microfilament-membrane interaction.