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 identity in their amino-terminal region to the membrane binding domain of erythrocyte band 4.1. Within this superfamily are three members, ezrin, radixin and moesin, that show approx. 75% overall sequence identity. A comparison of the domain structure and intracellular localization of ezrin and moesin in cultured cells is reported here. Limited proteolytic digestion of ezrin or moesin yields a relatively stable 32 kDa domain derived from the amino-terminal region that is homologous to the protease-resistant membrane binding domain of erythrocyte band 4.1. The remaining regions of the two proteins give rise to very different fragments, suggesting that the secondary/tertiary structures of the two proteins are different in these regions. We have generated polyclonal antibodies that discriminate between ezrin and moesin, and do not react with radixin. All cultured cell lines investigated contain ezrin, whereas moesin is variably expressed. Cells that contain both ezrin and moesin show a very similar pattern: both proteins are enriched and colocalize with actin in cell surface structures. Ezrin is also detected in the cytoplasm. In cells with few or no surface structures, both proteins show a patchy distribution in regions of the cell that contain fine networks of actin filaments. No staining of focal contacts or adherens 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, structure and ability to serve as a kinase substrate.