Direct involvement of ezrin/radixin/moesin (ERM)-binding membrane proteinsin the organization of microvilli in collaboration with activated ERM proteins

Ezrin/radixin/moesin (ERM) proteins have been thought to play a central rol e in the organization of cortical actin-based cytoskeletons including micro villar formation through cross-linking actin filaments and integral membran e proteins such as CD43, CD44, and ICAM-2. To examine the functions of thes e ERM-binding membrane proteins (ERMBMPs) in cortical morphogenesis, we ove rexpressed ERMBMPs (the extracellular domain of E-cadherin fused with the t ransmembrane/cytoplasmic domain of CD43, CD44, or ICAM-2) in various cultur ed cells. In cultured fibroblasts such as L and CV-1 cells, their overexpre ssion significantly induced microvillar elongation, recruiting ERM proteins and actin filaments. When the ERM-binding domains were truncated from thes e molecules, their ability to induce microvillar elongation became undetect able. In contrast, in cultured epithelial cells such as MTD-1A and A431 cel ls, the overexpression of ERMBMPs did not elongate microvilli. However, in the presence of EGF overexpression of ERMBMPs induced remarkable microvilla r elongation in A431 cells. These results indicated that ERMBMPs function a s organizing centers for cortical morphogenesis by organizing microvilli in collaboration with activated ERM proteins. Furthermore, immunodetection wi th a phosphorylated ERM-specific antibody and site-directed mutagenesis sug gested that ERM proteins phosphorylated at their COOH-terminal threonine re sidue represent activated ERM proteins.