Geometrical Analysis of F-actin Helix

Geometrical analyses are performed on the relationship between pitch length and rotational angle per monomer for F-actin helix, which is constructed by spherical monomeric units. The pitch length of 700 A and the number of monomers per turn, 26, observed in the electron micrograph by Hanson and Lowy, give rise to the diameter of 55 A by using the relation that each sphere must contact with neighboring four spheres, two in the same strand and the other two in the different strand, and the interaction sites on the monomer are the same for every monomer. A slight de-viation of the sites in position, however, results in a drastic change of helical dimension, e.g., movement of the sites by 0.3 A yields the change in pitch length as much as 30 A. A plausible helical dimension of F-actin helix is discussed considering the interaction with other muscle proteins, tropomyosin, troponin, and myosin