A globular subunit of microfilaments, highly conserved Actin is one of the most abundant proteins in eukaryotic cells. Monomeric actin, a slow ATPase composed of a 375 amino acid polypeptide chain, interacts with one molecule of ATP or ADP and contains high affinity and low affinity binding sites for divalent cations. Actin can quickly assemble into long microfilament polymers. In conjunction with myosin, actin is responsible for the contraction and relaxation of muscle. [ ]