DIFFERENTIAL REGULATION OF TROPOMYOSIN ISOFORM ORGANIZATION AND GENE-EXPRESSION IN RESPONSE TO ALTERED ACTIN GENE-EXPRESSION

Phenotypically altered C2 myoblast cells, generated by the stable tran sfection of human nonmuscle actin genes (Schevzov, G., C. Lloyd, and P . Gunning. 1992. J. Cell Biol. 117:775-786), exhibit a differential pa ttern of tropomyosin cellular organization and isoform gene expression . The beta-actin transfectants displaying a threefold increase in the cell surface area, showed no significant changes in the pattern of org anization of the high M(r) tropomyosin isoform, Tm 2, or the low M(r) tropomyosin isoform, Tm 5. In contrast, the gamma- and beta(sm)-actin gene transfectants, exhibiting a twofold decrease in the cell surface area, had an altered organization of Tm 2 but not Tm 5. In these actin transfectants, Tm 2 did not preferentially segregate into stress fibe r-like structures and the intensity of staining was greatly diminished . Conversely, a well-defined stress fiber-like organization of Tm 5 wa s observed. The pattern of organization of these tropomyosin isoforms correlated with their expression such that a profound decrease in Tm 2 expression was observed both at the transcript and protein levels, wh ereas Tm 5 remained relatively unchanged. These results suggest that r elative changes in nonmuscle actin gene expression can affect the orga nization and expression of tropomyosin in an isoform specific manner. Furthermore, this apparent direct link observed between actin and trop omyosin expression suggests that nonpharmacological signals originatin g in the cytoskeleton can regulate cytoarchitectural gene expression.