Mammalian cytoplasmic actins are the products of at least two genes and differ in primary structure in at least 25 identified positions from skeletal muscle actins.

Muscle and cytoplasmic actins from several species have been compared by extensive fingerprint analysis and by partial amino acid sequence determination with the known amino acid sequence of rabbit muscle actin. Although complete sequences have not been established, the following characteristics are apparent. (a) Cytoplasmic actins are the products of two different genes. The difference seen in isoelectric focusing studies is probably determined only by the nature of the three amino-terminal acidic residues. (b) Mammalian cytoplasmic actins are exceedingly similar and perhaps identical. (c) Cytoplasmic actins may differ by at least 25 amino acid replacement from rabbit muscle actin. These replacements have been identified for calf thymus actin; however, other cytoplasmic actins show the same replacements. (d) The replacements always involve-except for the first five residues-neutral amino acid residues. (e) The replacements are not randomly distributed. Residues 18-75 are constant whereas residues 2-18 and 259-298 show many substitutions. (f) The main component of smooth muscle actin from chicken gizzard shows the charge characteristics found at the amino terminus of the less acidic cytoplasmic actin species. In the rest of the polypeptide chain, gizzard actin resembles skeletal muscle actin, although two substitutions of the cytoplasmic type have been identified. (g) Heart muscle actin is very similar to skeletal muscle actin. Only two amino acid replacements have been found; they are of the cytoplasmic type. (h) Skeletal muscle actins from chicken and beef have not shown a replacement.