Summary

Expression of actin was examined in a cultured rat embryonic cell line 3Y1 and transformed cell lines that originated from 3Y1. An a-actin in addition to cytoplasmic ft- and y-actins was detected in 3Y1 by two-dimensional gel electrophoresis. This a-actin was hardly detected at all in the transformants induced by Rous sarcoma virus, v-H-ras oncogene or adenovirus type 12, while the a-actin was retained in the transformed cell lines induced by JV-methyl-JV-nitro-JV-nitrosoguanidine or in SV40, which are cell lines of relatively low malignancy. Western and Northern blot analyses established that this a-actin was a smooth muscle a-isofonn. An immunofluor-escence study revealed that smooth muscle a-actin in 3Y1 cells is present in stress fibers. Thus, smooth muscle a-actin is also a component of actin stress fibers, as /J- and y-actins are in 3Y1 cells. An alter-ation in the expression of this actin isoform may be related to phenotypical changes accompanying transformation. Key words: transformation, smooth muscle a-actin, 3Y1 fibroblasts, actin stress fibers