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 (α actin) was found only in differentiated muscle cells, and its synthesis is induced during myogenesis in culture. Two other forms (β and γ 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 nonmuscle actins are nearly identical. All three forms contain equimolar amounts of N^τ-methylhistidine, and extensive controls have shown no evidence of artifactual heterogeneity. In addition to the three major actins, two other proteins were identified as probable 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 <2 hr.