Ezrin: a protein requiring conformational activation to link microfilaments to the plasma membrane in the assembly of cell surface structures.

The cortical cytoskeleton of eucaryotic cells provides structural support to the plasma membrane and also contributes to dynamic processes such as endocytosis, exocytosis, and transmembrane signaling pathways. The ERM (ezrin-radixin-moesin) family of proteins, of which ezrin is the best studied member, play structural and regulatory roles in the assembly and stabilization of specialized plasma membrane domains. Ezrin and related molecules are concentrated in surface projections such as microvilli and membrane ruffles where they link the microfilaments to the membrane. The present knowledge about ezrin is discussed from an historical perspective. Both biochemical and cell biological studies have revealed that ezrin can exist in a dormant conformation that requires activation to expose otherwise masked association sites. Current results indicate that activated ezrin monomers or head-to-tail oligomers associate directly with F-actin through a domain in its C terminus, and with the membrane through its N-terminal domain. The association of ezrin with transmembrane proteins can be direct, as in the case of CD44, or indirect through EBP50. Other binding partners, including the regulatory subunit of protein kinase A and rho-GDI, suggest that ezrin is an integral component of these signaling pathways. Although the membrane-cytoskeletal linking function is clear, further studies are necessary to reveal how the activation of ezrin and its association with different binding partners is regulated.