Morphogenic effects of ezrin require a phosphorylation-induced transition from oligomers to monomers at the plasma membrane

ERM (ezrin, radixin, moesin) proteins act as linkers between the plasma mem brane and the actin cytoskeleton. An interaction between their NH2- and COO H-terminal domains occurs intramolecularly in closed monomers and intermole cularly in head-to-tail oligomers. In vitro, phosphorylation of a conserved threonine residue (T567 in ezrin) in the COOH-terminal domain of ERM prote ins disrupts this interaction. Here, we have analyzed the role of this phos phorylation event in vivo, by deriving stable clones producing wild-type, T 567A, and T567D ezrin from LLC-PK1 epithelial cells. We found that T567A ez rin was poorly associated with the cytoskeleton, but was able to form oligo mers. In contrast, T567D ezrin was associated with the cytoskeleton, but it s distribution was shifted from oligomers to monomers at the membrane. More over, production of T567D ezrin induced the formation of lamellipodia, memb rane ruffles, and tufts of microvilli. Both T567A and T567D ezrin affected the development of multicellular epithelial structures. Collectively, these results suggest that phosphorylation of ERM proteins on this conserved thr eonine regulates the transition from membrane-bound oligomers to active mon omers, which induce and are part of actin-rich membrane projections.