Actin binding site

The myosins are a superfamily of proteins that have the ability to convert energy released by ATP is hydrolysis into mechanical work. There are many forms of myosin, all of which have ATPase activity and an actin-binding site that is located... 

Huxley suggested that crossbridges can move out in this way and bind to actin because S-2 of HMM acted as a flexible link between LMM in the thick filament backbone and S-1. This was based on the observation that heavy meromyosin could be digested by chymotrypsin into two further subffagments (Lowey et al., 1966), S-1 and S-2, as described above, and that S-1 contained the ATPase and actin binding sites, whereas S-2 did not moreover, S-2 did not self-aggregate, as did the rod or LMM portion of myosin. 

Figure 49-4. Diagram of a myosin moiecuie showing the two intertwined a-heiices (fibrous portion), the giobuiar region or head (G),the iight chains (L), and the effects of proteoiytic cieavage by trypsin and papain. The giobuiar region (myosin head) contains an actin-binding site and an L chain-binding site and aiso attaches to the remainder of the myosin moiecuie.

Protein 4.1, a globular protein, binds tightly to the tail end of spectrin, near the actin-binding site of the latter, and thus is part of a protein 4.1-spectrin-actin ternary complex. Protein 4.1 also binds to the integral proteins, glycophorins A and C, thereby attaching the ternary complex to the membrane. In addition, protein 4.1 may interact with certain membrane phospholipids, thus connecting the lipid bilayer to the cytoskeleton. 

Iodoacetyl-LC-biotin has been used to localize the SH thiol of myosin by use of an avidin-biotin complex visualized by electron microscopy (Sutoh et al., 1984) and to determine the spatial relationship between SHj and the actin binding site on the myosin subfragment-1 surface (Yamamoto et al., 1984). 

Yamamoto, K., Sekine, T., and Sutoh, K. (1984) Spatial relationship between SHI and the actin binding site on myosin subfragment-1 surface. FEES Lett. 176, 75-78. 

Figure 4.5. Structure of myosin. Myosin comprises both light and heavy chains. The heavy chains may be cleaved by trypsin to generate light meromyosin (LMM) and heavy mero-myosin (HMM). Papain digestion of HMM yields subfragments SI and S2 each SI fragment contains an ATPase site and an actin-binding site. The light chains modify the activity of the ATPase.

One possesses the ATPase activity whereas the other possesses the actin-binding site. 

Figure 13.7 A diagram of the actin helix showing position of the tropomyosin. Both actin chains are flanked by tropomyosin molecules, which are long string-like molecules that span seven actin monomers. The troponin complex is attached to the tropomyosin but is not shown. From this diagram, it should be clear how the tropomyosin molecule can conceal the actin-binding sites for the myosin cross-bridges in the relaxed condition. A small conformational change in tropomyosin exposes the sites for attachment of the cross-bridges.

The decrease in the cytosolic Ca ion concentration reverses these changes, so that the actin-binding sites are concealed and the relaxed state of the muscle is restored. 

Within the N-terminal, ABDs of three actin-binding sites (ABS) from spectrin superfamily proteins have been delineated (ABS1, ABS2, and ABS3). The first and third ABS have been localized to the al helix in... 

Bresnick, A. R., Janmey, P. A., and Condeelis, J. (1991). Evidence that a 27-residue sequence is the actin-binding site of ABP-120. / Biol. Chem. 266, 12989-12993. 

Kuhlman, P. A., Hemmings, L., and Critchley, D. R. (1992). The identification and characterisation of an actin-binding site in alpha-actinin by mutagenesis. FEBS Lett. 304, 201-206. 

In the presence of low calcium concentration, Ca2+ binds to the S6 domain, induces release of the C- tail domain and frees the actin binding sites present in domains Sl-2. In order for severing to occur, higher concentrations of the ion are required. Additional calcium binding, most likely to domains SI, S2 and S4, fully activates the molecule, thus allowing severing to take place. 

Bryan, J. 1988. Gelsolin has three actin-binding sites. J Cell Biol. 106 1553-62. 

Way, M., B. Pope, and A. Weeds. 1991. Molecular biology of actin binding proteins evidence for a common structural domain in the F-actin binding sites of gelsolin and alpha-actinin. J Cell Sci Suppl. 14 91-4. 

When the structure in Fig. 29A is put into the fish muscle A-band unit cell (Fig. 29B note that the analysis of Hudson et al. [1997] defined the absolute orientation of the filament within the A-band lattice), it can be seen that the actin-binding sites on the myosi n heads are already close to... 

Stewart, M., and McLachlan, A. D. (1975). Fourteen actin-binding sites on tropomyosin Nature 257, 331—333. 

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