Tyrosine Phosphorylation of Moesin in Arachidonic Acid-Stimulated Human Platelets

Moesin, a member of the ezrin/radixin/moesin (ERM) family of cytoskeletal proteins, has been implicated in dynamic membrane-based processes such as the formation and stabilization of filopodia. Ezrin is known to be a substrate of tyrosine kinases in activated T cells and epithelial growth factor-stimulated A431 cells. For the closely related 77-kD protein moesin, which shares 72% identity with ezrin on the basis of their amino acid sequences, a reversible phosphorylation on tyrosine residues has not yet been described. Because our scanning electron microscopy studies revealed the appearance of multiple, up to 3 µm long filopodia on the surface of activated human platelets, we investigated the participation of moesin in dynamic shape changes on platelet stimulation with arachidonic acid. Antimoesin immunoprecipitates obtained under denaturing conditions from lysates of resting platelets contained only low amounts of tyrosine-phosphorylated moesin. In lysates of arachidonic acid-stimulated platelets, the level of tyrosine phosphorylation was significantly increased. This activation-dependent phosphorylation of moesin was verified by probing antiphosphotyrosine immunoprecipitates from unstimulated and stimulated platelets with antimoesin antibodies. Tyrosine-phosphorylated moesin was detectable only in the presence of the tyrosine phosphatase inhibitor vanadate, suggesting that a coordinated balance between kinase and phosphatase activities controls the steady-state level of moesin phosphorylation.