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© The Rockefeller University Press, 0021-9525/1999//1497 $5.00
The Journal of Cell Biology, Volume 145, Number 7, , 1999 1497-1509

Regular Articles

Direct Involvement of Ezrin/Radixin/Moesin (ERM)-binding Membrane Proteins in the Organization of Microvilli in Collaboration with Activated ERM Proteins



Shigenobu Yonemura*, Sachiko Tsukita*,{ddagger}, and Shoichiro Tsukita*

* Department of Cell Biology, Faculty of Medicine, Kyoto University, Kyoto 606-8501, Japan; and {ddagger} College of Medical Technology, Kyoto University, Kyoto 606, Japan

Ezrin/radixin/moesin (ERM) proteins have been thought to play a central role in the organization of cortical actin-based cytoskeletons including microvillar formation through cross-linking actin filaments and integral membrane proteins such as CD43, CD44, and ICAM-2. To examine the functions of these ERM-binding membrane proteins (ERMBMPs) in cortical morphogenesis, we overexpressed ERMBMPs (the extracellular domain of E-cadherin fused with the transmembrane/cytoplasmic domain of CD43, CD44, or ICAM-2) in various cultured cells. In cultured fibroblasts such as L and CV-1 cells, their overexpression significantly induced microvillar elongation, recruiting ERM proteins and actin filaments. When the ERM-binding domains were truncated from these molecules, their ability to induce microvillar elongation became undetectable. In contrast, in cultured epithelial cells such as MTD-1A and A431 cells, the overexpression of ERMBMPs did not elongate microvilli. However, in the presence of EGF, overexpression of ERMBMPs induced remarkable microvillar elongation in A431 cells. These results indicated that ERMBMPs function as organizing centers for cortical morphogenesis by organizing microvilli in collaboration with activated ERM proteins. Furthermore, immunodetection with a phosphorylated ERM-specific antibody and site-directed mutagenesis suggested that ERM proteins phosphorylated at their COOH-terminal threonine residue represent activated ERM proteins.

Key Words: ezrin • radixin • moesin • ERM • microvilli

Abbreviations used in this paper: CPERM, ERM protein phosphorylated at the COOH-terminal threonine residues; ERM, ezrin/radixin/ moesin; ERMBMP, ERM-binding membrane protein; pAb, polyclonal antibody; VSVG, vesicular stomatitis virus glycoprotein G.


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