A protein component of membranes isolated from 3T3 mouse fibroblasts and HeLa cells has been identified as actin by peptide mapping. Extensive but apparently not total coincidence was found between the peptide maps of these two nonmuscle membrane-associated actins compared to chick skeletal muscle actin. Between 2 and 4 percent of the total membrane protein appears in the actin band on sodium dodecyl sulfate polyacrylamide gels of 3T3 membranes while about 4 percent of the membrane protein appears as the actin band from HeLa membranes. These values represent approximately the same proportion of actin to total protein found in the cell homogenates. Treatment of intact cells with levels of cytochalasin B sufficient to cause pronounced morphological changes did not change the amount of actin associated with the membrane in either 3T3 or HeLa cells. However, incubation of isolated membranes under conditions favoring conversion of actin from filamentous to monomeric form resulted in dissociation of approximately 80 and 60 percent of the actin from 3T3 and HeLa membranes, respectively. Thus, approximately 20 percent of 3T3 membrane actin and 40 percent of HeLa membrane actin remained associated with the membrane even under actin depolymerizing conditions.

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