CELLULAR DISTRIBUTIONS OF THE ERM PROTEINS IN MDCK EPITHELIAL CELLS: REGULATION BY GROWTH AND CYTOSKELETAL INTEGRITY

Abstract

The highly homologous ERM (ezrin/radixin/moesin) proteins, molecular cross-linkers which connect the cell membrane with the underlying cytoskeleton, have molecular weights of 81, 80 and 78kDa respectively. We present data which shows significant variation in the molecular weight and presence of multiple forms of ERM proteins in different cell lines, such that specific antibodies to each protein are essential for unambiguous detection. Biochemical fractionation of MDCK cells demonstrates that although the individual ERM fractionation patterns are unaltered by cell density, the multiple forms of moesin each associate with different subcellular fractions. Since ERM proteins can exist in dormant or active conformations corresponding to their phosphorylation state, we propose that the partitioning of ERM proteins between subcellular compartments may depend on their activation status. In addition, we show that when the co-localization between ezrin and F-actin is disrupted by cytochalasin D, MDCK cells undergo a dramatic morphology change during which long, branching, ezrin-rich protrusions are formed. Consistent with other workers, our data suggest that maintenance of ezrin:F-actin interactions are required for the maintenance of normal cellular morphology.