Actin exists in two forms: G-actin (or globular actin) and F-actin (or fibrous actin). G-actin, a single polypeptide chain with a molecular weight of about 42 kDa, has a roughly globular configuration. ... G-actin also has one ATP binding site per monomer. F-actin is a filamentous polymer, composed of G-actin monomers.