EZRIN CONTAINS CYTOSKELETON AND MEMBRANE-BINDING DOMAINS ACCOUNTING FOR ITS PROPOSED ROLE AS A MEMBRANE-CYTOSKELETAL LINKER

Ezrin, a widespread protein present in actin-containing cell-surface structures, is a substrate of some protein tyrosine kinases. Based on its primary and secondary structure similarities with talin and band 4.1 it has been suggested that this protein could play a role in linking the cytoskeleton to the plasma membrane (Gould, K. L., A. Bretscher, F. S. Esch, and T. Hunter. 1989. EMBO (Eur. Mol. Biol. Organ.), J. 8:41334142; Turunen, O., R. Winqvist, R. Pakkanen, K.-H. Grzeschik, T. Wahlstrom, and A. Vaheri. 1989. J. Biol. Chem. 264:16727-16732). To test this hypothesis, we transiently expressed the complete human ezrin cDNA, or truncated cDNAs encoding the amino- and carboxy-terminal domains of the protein, in CV-1 cells. Protein epitope tagging was used to unambiguously determine the subcellular distribution of the protein encoded by the transfected cDNA. We show that this protein is concentrated underneath the dorsal plasma membrane in all actin-containing structures and is partially detergent insoluble. The amino-terminal domain displays the same localization but is readily extractable by nonionic detergent. The carboxy-terminal domain colocalizes with microvillar actin filaments as well as with stress fibers and remains associated with actin filaments after detergent extraction, and with disorganized actin structures after cytochalasin D treatment. Our results clearly demonstrate that ezrin interacts with membrane-associated components via its amino-terminal domain, and with the cytoskeleton via its carboxy-terminal domain. The ammo-terminal domain could include the main determinant that restricts the entire protein to the cortical cytoskeleton in contact with the dorsal plasma membrane and its specialized microdomains such as microvilli, microspikes and lamellipodia.