MOESIN, LIKE EZRIN, COLOCALIZES WITH ACTIN IN THE CORTICAL CYTOSKELETON IN CULTURED-CELLS, BUT ITS EXPRESSION IS MORE VARIABLE

The band 4.1 superfamily of proteins show approx. 30% sequence identit y in their amino-terminal region to the membrane binding domain of ery throcyte band 4.1. Within this superfamily are three members, ezrin, r adixin and moesin, that show approx. 75% overall sequence identity. A comparison of the domain structure and intracellular localization of e zrin and moesin in cultured cells is reported here. Limited proteolyti c digestion of ezrin or moesin yields a relatively stable 32 kDa domai n derived from the amino-terminal region that is homologous to the pro tease-resistant membrane binding domain of erythrocyte band 4.1. The r emaining regions of the two proteins give rise to very different fragm ents, suggesting that the secondary/tertiary structures of the two pro teins are different in these regions. We have generated polyclonal ant ibodies that discriminate between ezrin and moesin, and do not react w ith radixin. All cultured cell lines investigated contain ezrin, where as moesin is variably expressed. Cells that contain both ezrin and moe sin show a very similar pattern: both proteins are enriched and coloca lize with actin in cell surface structures. Ezrin is also detected in the cytoplasm. In cells with few or no surface structures, both protei ns show a patchy distribution in regions of the cell that contain fine networks of actin filaments. No staining of focal contacts or adheren s junctions was observed. These results, together with those of others , lead to the conclusion that, of the members of this protein family, only radixin is an authentic component of adherens junctions and focal contacts. Ezrin and moesin are both found in cell surface structures after treatment of human A431 cells with epidermal growth factor, and ezrin, but not moesin, becomes phosphorylated on tyrosine. This study shows that ezrin and moesin have a similar subcellular distribution in cultured cells, yet are distinguishable in their expression, structur e and ability to serve as a kinase substrate.