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'''''Structure'''''

Actin exists in both its monomeric (globular, G-actin) and polymeric (filamentous or F-actin) form. The actin monomer is approximately pear shaped. Actin is composed of 376 residues that are folded into two large domains, each comprised of two subdomains. Its secondary structure is 41% helical (22 helices; 153 residues) and 20% beta sheet (20 strands; 77 residues). (PDB) The actin filament is a double-stranded, right-handed helix with a half-pitch of 37 nm and a one-start left-handed genetic helix with a rise of 2.75 nm per monomer. The width of the filament is within the range of 7–10 nm. (Conformational Dynamics of Actin: Effectors and Implications for Biological Function) The large domains are organized to form a hinged molecule with a deep cleft. Subdomains 1 and 3 are structurally related, whereas subdomains 2 and 4 can be viewed as large insertions into subdomains 1 and 3, respectively. There is moderately little contact between the two major domains of actin; the polypeptide chain passes twice between these domains at the loop centered at residue Lys336 and at the linker helix Gln137-Ser145, which functions as the hinge axis between the domains. As a result, two clefts are formed between the domains. Within the upper cleft are the cofactors, an adenine nucleotide and a divalent metal ion, usually magnesium. It is thought that the cofactors interact with the domains on either side, increasing their connectivity. The lower cleft between domains 1 and 3 is lined by residues Tyr143, Ala144, Gly146, Thr148, Gly168, Ile341, Ile345, Leu346, Leu349, Thr351, and Met355, which are predominantly hydrophobic. This cleft constitutes the major binding site for most ABPs, and is thus called the targetbinding or hydrophobic cleft. (Actin Structure and Function). Most of the interaction that occur in the subunit are electrostatic in character but there are also hydrophobic interactions.  (Actin Structure and Function)