Identification and characterization of multiple forms of actin.

Multiple forms of actin have been found in a variety of mammalian cell lines and tissues by the use of high resolution, two-dimensional gel electrophoresis. One form (alpha actin) was found only in differentiated muscle cells, and its synthesis is induced during myogenesis in culture. Two other forms (beta and gamma actin) are present in all nonmuscle cell types examined, and they continue to be synthesized in cultured muscle cells after fusion. Tryptic peptide comparisons have shown that muscle actin is distinguished from the two "nonmuscle" actins by several peptide differences, and that the two non-muscle actins are nearly identical. All three forms contain equimolar amounts of N-methylhistidine, and extensive controls have shown no evidence of artifactual heterogeneity. In addition to the three major actins, two other proteins were identified as probably forms of actin by affinity for DNAase I-agarose. These proteins are similar in charge and molecular weight to the major actin forms, but are unstable and have lifetimes in the cell of less than 2 hr.