Vertebrates express three main actin isoforms, including three α-isoforms of skeletal, cardiac, and smooth muscles and the β- and γ-isoforms expressed in nonmuscle and muscle cells. Actin isoforms differ by only a few amino acids, with most variations occurring toward the N terminus (23
). Actin also undergoes various forms of posttranslational modifications. For instance, His73 of skeletal muscle α-actin is methylated, the N-terminal methionine and cysteine residues are acetylated and cleaved, and the resulting N-terminal aspartic acid is then reacetylated.
Since the original determination of the crystal structure of G-actin in complex with DNase I (28
), over 80 structures of actin have been reported (
). These are listed in Supplemental Table 1
(follow the Supplemental Material link from the Annual Reviews home page at http://www.annualreviews.org
). The majority of these structures have been obtained as complexes with actin-binding proteins (ABPs) and small molecules, or by chemically modifying or mutating actin in order to prevent polymerization. Remarkably, irrespective of the bound molecule or nucleotide state, the conformation of the actin monomer is basically the same. Actin belongs to a structural superfamily with sugar kinases, hexokinases, and Hsp70 proteins (3
). The Arp proteins (49
) and the prokaryotic actin-like proteins MreB (54
) and ParM (56
) are also now part of this superfamily. Common to these proteins, the 375-amino-acid (aa) polypeptide chain of actin folds into two major α/β-domains (
). Because of their location within the actin filament, the two major domains of actin are known as the outer and inner domains, and because of their apparently different sizes in electron microscopy (EM) images, they have also been called the small and large domains, respectively. Traditionally, however, a four-subdomain nomenclature has been adopted (28
). Subdomains 1 and 3 are structurally related and probably emerged from gene duplication, whereas subdomains 2 and 4 can be viewed as large insertions into subdomains 1 and 3, respectively. For brevity we refer to these as domains 1--4.The actin monomer is rather flat, fitting into a rectangular prism with dimensions 55 Å × 55 Å × 35 Å.
Figure 1 Structures of actin and actin complexes. The structures of actin complexes are shown to scale and in chronological order of publication. (a) Classical view of the structure of the actin monomer. The structure shown was derived from the complex with DNase (more ...)
bThere is relatively little contact between the two major domains of actin; the polypetide chain passes twice between these domains: at the loop centered at residue Lys336 and at the linker helix Gln137-Ser145, which functions as the axis of a hinge between the domains. As a result, two clefts are formed between the domains. The upper cleft binds the nucleotide and associated divalent cation (Mg2+
in cells), which together provide the other important linkage between domains (
). 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 also mediates important longitudinal contacts between actin subunits in the filament (16, 36
), as suggested previously (9
). The communication between the two clefts provides the structural basis for how nucleotide-dependent conformation changes modulate the binding affinities of ABPs and the strength of intersubunit contacts in the filament.
G-actin is not an effective ATPase, whereas F-actin is and most crystal structures have been solved in the ATP-bound form. The differences between the ATP- and ADP-bound states are relatively minor and involve primarily two loops (
): the Ser14 β-hairpin loop and the sensor loop carrying the methylated His73 (21
). The Ser14 loop is located in actin subdomain 1 and is structurally equivalent to the loop containing Asp157 in subdomain 3. These two loops engulf the phosphates of the nucleotide. Nucleotide-dependent conformational changes begin with Ser14, which in the ATP state makes a hydrogen-bonding contact with the γ-phosphate. After hydrolysis and γ-phosphate release, Ser14 changes orientation to form a hydrogen-bonding contact with the β-phosphate of the nucleotide. In the ATP state, Ser14 also forms a hydrogen-bonding contact with the main chain of the loop carrying His73. When the side chain of Ser14 rotates in the ADP state, the His73 loop moves toward the nucleotide to occupy some of the space emptied by the γ-phosphate. In this way, this loop appears to sense the state of the nucleotide.
The sensor loop marks the C-terminal end of domain 2, which can be viewed as an insertion into domain 1. At the top of domain 2, residues 39--51 are disordered in most crystal structures. This sequence is referred to as the DNase I-binding loop (or D-loop) because it mediates important interactions in the complex with DNase I (28
). When visible in crystal structures this loop takes on a variety of confirmations, including in one case a short a-helix (40
). Domain 2, and specifically the D-loop, plays a critical role in longitudinal intersubunit contacts in the filament. Thus, changes in the Ser14 and sensor loops appear to propagate to domain 2, whose conformation changes only slightly between nucleotide states (21
), but probably enough to explain the decreased stability of the actin filament in the ADP state. Because the nucleotide sits at the interface between the two major actin domains, another important consequence of nucleotide hydrolysis and γ-phosphate release is a weakening of the linkage between domains, which can then rotate more freely with respect to one another. Two types of rotations have been described: a scissors-like opening and closing of the nucleotide cleft within the plane of the figure (
) and a perpendicular propeller-like rotation of one domain with respect to the other (18
). Gln137, at the beginning of the hinge helix, is thought to play a critical role in nucleotide hydrolysis (27
), which is activated in the filament (7
), presumably by interdomain rotation that brings the side chain of Gln137 in contact with the γ-phosphate of the nucleotide (16