Morphogenic Effects of Ezrin Require a Phosphorylation-Induced Transition from Oligomers to Monomers at the Plasma Membrane

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Abstract
ERM (ezrin, radixin, moesin) proteins act as linkers between the plasma membrane and the actin cytoskeleton. An interaction between their NH2- and COOH-terminal domains occurs intramolecularly in closed monomers and intermolecularly in head-to-tail oligomers. In vitro, phosphorylation of a conserved threonine residue (T567 in ezrin) in the COOH-terminal domain of ERM proteins disrupts this interaction. Here, we have analyzed the role of this phosphorylation event in vivo, by deriving stable clones producing wild-type, T567A, and T567D ezrin from LLC-PK1 epithelial cells. We found that T567A ezrin was poorly associated with the cytoskeleton, but was able to form oligomers. In contrast, T567D ezrin was associated with the cytoskeleton, but its distribution was shifted from oligomers to monomers at the membrane. Moreover, production of T567D ezrin induced the formation of lamellipodia, membrane 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 threonine regulates the transition from membrane-bound oligomers to active monomers, which induce and are part of actin-rich membrane projections.

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