Actin is one of the principal proteins in the muscle. Along with myosin, it is involved in the process of muscle contraction.
Actin occurs in two forms, G-actin (globular actin) and F-actin (fibrous actin), which is a polymer of G-actin. The monomeric G-actin polymerizes to F-actin in the presence of salts and ATP. Rabbit muscle G-actin has been extensively studied. It is globular in shape, consists of a single polypeptide chain and has a molecular weight of 46,000.
Actin itself has no catalytic activity. But, when polymerized, it is capable of stimulating the ATPase activity of myosin.

The purity of actin is assessed by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate after initial staining with coomassie blue. The product has a purity of greater than 90% as judged by this method.