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.