Plasma membrane-Actin filament Association through ERM Family Members

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The ERM family members, ezrin, radixin, and moesin, localizing just beneath the plasma membranes are thought to be involved in the actin filament/plasma membrane association. To identify the integral membrane protein directly associated with ERM family members, we performed immunoPrecipitaion studies using anti-moesin mAb and cultured BHK cells metabol-ically-labeled with [^<35>S] methionine or surface-labeled with biotin. The results indicated that moesin is directly associated with a 140kD integral membrane protein. Using BHK cells as antigens, we obtained a mAb which recognized the 140kD membrane protein. We next cloned a cDNA encoding the 140kD membrane protein and identified it as CD44, a broadly-distributed cell surface glycoprotein. Immunoprecipitation with various anti-CD44 mAbs showed that ezrin and radixin as well as moesin are associated with CD44 not only in BHK cells but also in mouse L fibroblasts. Furthermore, immunofluorescence microscopy revealed that in both BHK and L cells, the Triton X-100-insoluble CD44 is precisely colocalized with ERM family members. We concluded that ERM family members work as molecular linkers between the cytoplasmic domain of CD44 and actin-based cytoskeletons.