OE-Actinin is an actin binding and crosslinking protein found in both muscle and non-muscle cells at points where actin is anchored to a variety of intracellular structures. It is a dimer, probably a homodimer, with a subunit molecular weight of 94-103 kDa (Endo & Masaki, 1982; Landon et al., 1985; Baron et al., 1987a), the subunits being anti-parallel in orientation (Wallraff et al., 1986; Imamura et al., 1988). It is visualized as a long rod-shaped molecule in the electron microscope, 3-4nm wide by 30-40nm in length (Podlubnaya et al., 1975; Bretscher et al., 1979; Imamura et al., 1988). A number of distinct isoforms of 0c-actinin have been characterized including the skeletal and smooth muscle isoforms (Endo & Masaki, 1982), and the non-muscle isoforms isolated from brain (D~ ihaiman & Bamburg, 1984), macrophages (Bennett et al., 1984), platelets (Landon et al., 1985) and cultured fibroblasts (Burridge & Feramisco, 1981). The only clear functional difference between these isoforms is that binding of the various nonmuscle~-actinins to actin is calcium sensitive, whereas binding of the muscle isoforms is calcium insensitive (Burridge & Feramisco, 1981).