Actin monomer subdomains

How does actin bind The actin monomer consists of four subdomains, 1, 2, 3, and 4 numbered from the N terminus (Fig. 7-10). The negatively charged N-terminal region of actin contains the sequence... 

Fig. 8. Illustration of the actin monomer structure solved by X-ray crystallography (Kabsch et al, 1990) showing the four structural subdomains of the actin monomer, labeled, in the front (A) and back (B) view with N, C-termini labeled 1-4. Also labeled is the loop linking residues 262 and 309 between subdomains 3 and 4.

Fig. 9. (A) Actin monomer with each of its four structural subdomains shown in...

As seen earlier (in Fig. 5), it is the lattice shape and size in a crystal that define the location of the diffraction spots and it is the repeating unit on the lattice that defines the relative intensities of these spots. So it is with actin filaments. It is the symmetry of the array of actin subunits that determines where the layer lines will be (as in Fig. 10), but it is the shape and orientation of the actin subunits on each point along the helix that define the relative intensities of these actin layer lines. The actin monomer (as discussed in Squire et al. in this volume see Section II.A.3 and Fig. 9) has four subdomains in which subdomains 3 and 4 lie close to the helix axis and subdomains 1 and 2 are on the outside of the helix. Subdomain 1 is relatively large and is where myosin heads bind, whereas subdomain 2 is relatively small and its precise role is not clear. However, it can be shown (e.g., Harford and Squire, 1997) that even quite small movements of subdomain 2 can have their effect on the intensities of the low-angle actin layer lines. In terms of resolution, which is often all-important in structural... 

The refined structure of the F-actin monomer was found to vary from G-actin primarily in two ways. First, the large and small domains (each of which is comprised of two of the subdomains) adjusted their orientation relative to one another. Second, residues 230-250 in subdomain 4 shifted to close the nucleotide binding cleft. The normal mode refinement approach was therefore able to determine a model for F-actin which derived from low energy distortions to the G-actin structure and minor reorientation of its four subdomains and which fit the diffraction data quite well using a small number of adjustable parameters. 

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