Unique isoactins in the brush border of rat intestinal epithelial cells

The mammalian genome contains 20-30 genes encoding a family of actins. To date, however, only six proteins (four muscle and two nonmuscle isoforms) encoded by this multigene complex have been identified. We have isolated two actins from the brush border of rat intestinal epithelial cells that have isoelectric points and N-terminal peptides characteristic of the cytoplasmic beta- and gamma-actins. However, using a panel of actin-specific monoclonal antibodies, we show that these actins contain a set of epitopes that distinguishes them from any of the known cytoplasmic or muscle isoforms. These unique actins share features of both the nonmuscle and muscle isoforms, suggesting that they represent an intermediate in the evolution of the specialized muscle actins.