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

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