Structural conversion between open and closed forms of radixin: Low-angle shadowing electron microscopy

The function of ERM (ezrin/radixin/moesin) proteins as general cross-linker s between actin filaments and plasma membranes is regulated downstream of R ho, through the transition between active and inactive forms. To directly e xamine the conformational change between the active and inactive forms of E RM proteins, we applied low-angle rotary-shadowing electron microscopy to t he radixin molecules, wild-type, T564A-non-phosphorylated-type, and T564E-p hosphorylated-type, since most of the active forms are reportedly stabilize d in cells by the C-terminal threonine phosphorylation. As a result, the T5 64A- and wild-type radixin molecules yielded the globular closed forms, sim ilar to8-14 nm in diameter, with some striations on their surfaces. In cont rast, the T564E-radixin molecules tended to take elongated open forms, in w hich two globular structures measuring similar to8 nm and similar to5 nm in diameter were associated with both ends of the filamentous structures. The filamentous structure took either a similar to 20-25 nm-long straight cour se or a folded course. Taken together with the biochemical and the crystal structural results obtained to date, the closed and open forms represent th e inactive and active forms of radixin as cross-linkers between actin filam ents and plasma membranes. (C) 2001 Academic Press.