Functional alterations in beta'-actin from a KB cell mutant resistant to cytochalasin B.

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Abstract
We recently described the isolation of mutant KB cells (Cyt 1 cells) resistant to the cytotoxic effect of cytochalasin B (CB). This mutant carried an altered .beta.-actin; i.e., .beta.''-actin (Toyama, A., and S. Toyama. 1984. Cell. 37:609-614). In the present study, we have examined the functional properties of actin in Cyt 1 cells. Our results showed that increased resistance of Cyt 1 cells to CB was reflected in altered properties of .beta.''-actin itself. This was shown directly by two findings. First, the polymerization of .beta.''-actin was more resistant than that of .beta.- or .gamma.-actin to the multiple effect of CB. Second, .beta.''-actin bound less CB than .beta.- or .gamma.-actin. The functional alteration of .beta.''-actin in Cyt 1 cells was further supported by the observation that, although treatment of KB cells with CB increased the pool of unpolymerized actin, the same treatment did not affect the pool of unpolymerized actin in Cyt 1 cells, and that microfilaments of Cyt 1 cells were more resistant to the disrupting action of CB that those of KB cells. These results strongly suggest that the primary site of actin of CB on cell motility processes is actin.