alpha-Skeletal actin induces a subset of muscle genes independently of muscle differentiation and withdrawal from the cell cycle

Muscle differentiation is characterized by the induction of genes encoding contractile structural proteins and the repression of nonmuscle isoforms fr om these gene families. We have examined the importance of this regulated o rder of gene expression by expressing the two sarcomeric muscle actins char acteristic of the differentiated state, i.e. alpha -skeletal and alpha -car diac actin, in C2 mouse myoblasts, Precocious accumulation of transcripts a nd proteins for a group of differentiation-specific genes was elicited by a lpha -skeletal actin only: four muscle tropomyosins, two muscle actins, des min and MyoD, The nonmuscle isoforms of tropomyosin and actin characteristi c of the undifferentiated state continued to be expressed, and no myosin he avy or light chain or troponin transcripts characteristic of muscle differe ntiation were induced, Stable transfectants displayed a substantial reducti on in cell surface area and in the levels of nonmuscle tropomyosins and bet a -actin, consistent with a relationship between the composition of the act in cytoskeleton and cell surface area. The transfectants displayed normal c ell cycle progression. We propose that alpha -skeletal actin can activate a regulatory pathway linking a subset of muscle genes that operates independ ently of normal differentiation and withdrawal from the cell cycle.