SOLUBLE EZRIN PURIFIED FROM PLACENTA EXISTS AS STABLE MONOMERS AND ELONGATED DIMERS WITH MASKED C-TERMINAL-EZRIN-RADIXIN-MOESIN ASSOCIATIONDOMAINS

Previous work has indicated that ezrin, a membrane-microfilament linki ng protein, exists largely as a monomeric protein in solution. Here we purify from human placenta two cytosolic ezrin species that chromatog raph differently on gel filtration, anion, and cation exchange resins. Both species contain only the ezrin polypeptide, yet they do not read ily interconvert in vitro as determined by gel filtration analysis. De termination of the physical properties of the two species indicates th at one represents the conventional monomer, whereas the other represen ts highly asymmetric dimers. Chemical crosslinking data support this c onclusion. Purified ezrin monomers normally have a masked C-terminal d omain (termed a C-ERMAD) that, upon exposure, can associate with an N- terminal domain (termed N-ERMAD) of another ezrin molecule. Here we sh ow that purified ezrin dimers also have masked C-ERMADs. On the basis of these results, we suggest a working model for the molecular organiz ation of ezrin monomers and dimers and propose a hypothesis that expla ins the stable coexistence of ezrin monomers and dimers in placenta. S ince 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 radixi n and moesin as well.