Actin, alpha-actinin, and tropomyosin interaction in the structural organization of actin filaments in nonmuscle cells.

Abstract

During the spreading of a population of rat embryo cells, approximately 40% of the cells develop a strikingly regular network which precedes the formation of the straight actin filament bundles seen in the fully spread out cells. Immunofluorescence studies with antibodies specific for the skeletal muscle structural proteins actin, oz-actinin, and tropomyosin indicate that this network is composed of foci containing actin and a-actinin, connected by tropomyosin-associated actin filaments. Actin filaments, having both tropomyosin and a-actinin associated with them, are also seen to extend from the vertices of this network to the edges of the cell. These results demonstrate a specific interaction of a-actinin and tropomyosin with actin filaments during the assembly and organization of the actin filament bundles of tissue culture cells. The three-dimensional network they form may be regarded as the structural precursor and the vertices of this network as the organization centers of the ultimately formed actin filament bundles of the fully spread out cells.

Localization of [alpha]-actinin and tropomyosin in muscle and nonmuscle systems

“…Although the results described in this thesis for anti-tropomyosin and anti-a-actinin binding to chick embryo fibroblasts are similar to the results reported by Lazarides and coworkers (Lazarides, 1975a(Lazarides, , 1976Lazarides and Burridge, 1975), some subtle but important differences exist. Periodic binding of anti-a-actinin used in the present studies produced fluorescent segments 150 to 200 nm long and nonfluorescent segments 1100 to 1900 nm in length compared with 300 to 500 and 800 to 1700 nm segments reported for the same binding by Lazarides and Burridge (1975).…”

“…Lazarides (1976) and Lazarides and Burridge (1975) find less anti-a-actinin binding at the periphery of cells, show periodic binding of a-actinin more frequently, and have less anti-a-actinin binding to stress fibers than was found in the present study. Although these subtle differences in the pattern of anti-a-actinin binding to fibroblasts could originate from differences in handling of the cells, it also seems possible that the a-actinin antigen used by Lazarides was not homogeneous.…”

“…The ostensible length of tropomyosin molecules with subunit molecular weights of 30,000 to 31,000 daltons is 340 to 345 A (Fine ^ , 1973). Assuming the same geometrical relationship to the double-stranded actin filament as exists in skeletal, Lazarides and coworkers (Lazarides, 1975a(Lazarides, , 1976Lazarides and Burridge, 1975) have shown that anti-a-actinin and anti-tropomyosin bind in a periodic fashion to some, but not all, of the microfilaments in fibro blasts. Lazarides (1975a) finds that anti-tropomyosin binds to some microfilaments in human skin fibroblasts to produce fluorescent segments 800 to 1700 nm long (average length of 1200 nm) and nonfluorescent seg ments 300 to 500 nm long (average length of 400 nm).…”

“…It is interesting that anti-a-actinin also binds to the nucleus (Figure 54b), Lazarides (1975bLazarides ( , 1976 has pre viously noted that anti-a-actinin binds to nuclei. It seems possible that anti-a-actinin binding to nuclei may be related to the recent findings that much of the nonhistone protein in nuclei consists of contractile muscle protein (Douvas £t al., 1975;Jockush et al, 1974;Lestourgeon et al, 1975).…”

“…Tlie results described in this thesis show that this a-actinin antigen is inhomogeneous and elicits antibodies that bind to M-lines dd well as to Z-disks of myofibrils. Moreover, Lazarides (1976) finds a "split-line" effect upon immunoelectrophoresis of his anti-a-actinin against the antigenic a-actinin used to produce it. We have shown (Schollmeyer _et al, 1976) that antibody to a 170,000-dalton M-protein binds periodically to chick embryo fibroblasts and that anti-M-protein, anti-a-actinin, and anti-tropomyosin all are necessary to produce a continuous fluorescence along microfilaments in chick embryo fibroblasts.…”