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J. Cell Biol.,
Volume 145, Number 7, June 28, 1999 1497-1509
* Department of Cell Biology, Faculty of Medicine, Kyoto University, Kyoto 606-8501, Japan; and 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.
College of Medical
Technology, Kyoto University, Kyoto 606, Japan
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