Dynamic association of moesin with the membrane skeleton of thrombin- activated platelets.

In the June 15, 1998 issue of Blood, Serrador et al1 report on redistribution of ezrin and moesin to the uropod of polarized T lymphoblasts, suggesting their role in establishing cell-cell contacts. The ERM (ezrin-radixin-moesin) family proteins colocalize in nonhematopoietic cells with actin filaments in surface projections, microvilli, microspikes, filopodia, ruffles, etc, where they function in regulated linkage of plasma membrane proteins with actin in the cytoskeleton.2,3 Cells contain soluble pools of ERM monomers, which are dormant due to intramolecular association of their Nand C-terminal regions.4 Phosphorylation and other activation reactions conformationally unmask binding sites and promote assembly of target-associated oligomeric ERM structures.2,3 Although ezrin is the most broadly expressed ERM protein, moesin is quantitatively dominant in leukocytes5 and is the only ERM protein in platelets.6 When smooth surfaced circulating platelets are stimulated to participate in hemostasis, they undergo rapid cytoskeletal rearrangements, developing filopodia and ruffling lamellae. To better understand the role of ERM proteins in blood cells, we used established approaches to determine moesin localization in resting and thrombin-activated platelets. Immunofluorescent microscopy showed moesin localized at the periphery of resting platelets with the central cytoplasmic cores essentially unstained (Fig 1A, first panel). One minute after thrombin addition, moesin became localized in newly formed filopodial and lamellipodial projections, and double staining showed filamentous actin also localized