Actin Binding and Head-to-Tail Polymerization

14-fold periodicity of acidic and outer nonpolar residues (excluding those in the core a and d positions). Basic residues are more randomly distributed. Statistically the repeat was about 19.66 residues, and since the effective length of each chain of the molecule in a head-to-tail association of TM molecules may be taken 

FIGURE 6 Two TM molecules are shown linked head-to-tail as observed in the X-ray crystal structure. Note the right-handed supercoil with a small variable radius. When bound to actin, each TM is believed to be supercoiled with a radius of about 40 A. The projected molecular length is reduced to —385 A. With a pitch of —140 A, each half turn of the TM coiled-coil can make equivalent interactions with each of seven actin monomers. The structure of the head-to-tail joint is unknown but is believed to be a globular domain. This representation is reproduced from Fig. 11 of Phillips et al. /. Mol. Biot. 192, 111-131 (1986) by permission of Academic Press. 

Phillips has reexamined this question (Phillips etal., 

1986) by taking into consideration the molecular parameters of the coiled-coil as determined in the crystal structure, the azimuthal positions of the amino acid side chains, and how these could be presented to each actin monomer along the F-actin strand (see Figs. 12 and 13 of Phillips et al., 1986). Each of the a zones was 

7-fold periodicity for actin binding, but did confirm the importance of such a periodicity. 

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