Soluble ezrin purified from placenta exists as stable monomers and elongated dimers with masked C-terminal ezrin-radixin-moesin association domains

Previous work has indicated that ezrin, a membrane-microfilament linking protein, exists largely as a monomeric protein in solution. Here we purify from human placenta two cytosolic ezrin species that chromatography differently on gel filtration, anion, and cation exchange resins. Both species contain only the ezrin polypeptide, yet they do not readily interconvert in vitro as determined by gel filtration analysis. Determination of the physical properties of the two species indicates that one represents the conventional monomer, whereas the other represents highly asymmetric dimers. Chemical cross-linking data support this conclusion. Purified ezrin monomers normally have a masked C-terminal domain (termed a C-ERMAD) that, upon exposure, can associate with an N-terminal domain (termed N-ERMAD) of another ezrin molecule. Here we show that purified ezrin dimers also have masked C-ERMADs. On the basis of these results, we suggest a working model for the molecular organization of ezrin monomers and dimers and propose a hypothesis that explains the stable coexistence of ezrin monomers and dimers in placenta. Since radixin and moesin, the two other members of the closely related ERM protein family, both contain N- and C-ERMADs, the results we have documented and models proposed for ezrin are likely to apply to radixin and moesin as well.