EZRIN HAS A COOH-TERMINAL ACTIN-BINDING SITE THAT IS CONSERVED IN THEEZRIN PROTEIN FAMILY

Ezrin, previously also known as cytovillin, p81, and 80K, is a cytopla smic protein enriched in microvilli and other cell surface structures. Ezrin is postulated to have a membrane-cytoskeleton linker role. Rece nt fmdings have also revealed that the NH2- terminal domain of ezrin i s associated with the plasma membrane and the COOH-terminal domain wit h the cytoskeleton (Algrain, M., O. Turunen, A. Vaheri, D. Louvard, an d M. Arpin. 1993. J. Cell Biol. 120: 129-139). Using bacterially expre ssed fragments of ezrin we now demonstrate that ezrin has an actin-bin ding capability. We used glutathione-S-transferase fusion proteins of truncated ezrin in affinity chromatography to bind actin from the cell extract or purified rabbit muscle actin. We detected a binding site f or filamentous actin that was localized to the COOH-terminal 34 amino acids of ezrin. No binding of monomeric actin was detected in the assa y. The region corresponding to the COOH-terminal actin-binding site in ezrin is highly conserved in moesin, actin-capping protein radixin an d EM10 protein of E. multilocularis, but not in merlin/schwannomin. Co nsequently, this site is a potential actin-binding site also in the ot her members of the protein family. Furthermore, the actin-binding site in ezrin shows sequence homology to the actin-binding site in the COO H terminus of the p subunit of the actin-capping protein CapZ and one of the potential actin-binding sites in myosin heavy chain. The actin- binding capability of ezrin supports its proposed role as a membrane-c ytoskeleton linker.