A rotational offset model for two-stranded F-actin.

We propose the following "rotational offset" model for two independent strands of F-actin to account for the observation that it is possible, at times, for the crossover repeat to either alternate between long and short periods or remain constant (Bremer et al., 1991. J. Cell Biol. 115, 689-703). Rotational offset is the manifestation of the angular component of "lateral slipping" between the two long-pitch, right-handed strands comprising the actin filament. The present model is based on the premise that the longitudinal bond connecting the subunits along a single long-pitch strand is substantially stronger than the diagonal bond that connects interstrand subunits. We pose that, over fairly long stretches, the backbones of the two right-handed strands are individually close to being ideal helices, and that it is possible for the backbone of one strand to "roll across" the other. The rotational offset angle (epsilon 0) is the amount that one helical strand is angularly displaced relative to the position that otherwise would allow the two strands to be described as an ideal single genetic helix. Such an independent movement of the two strands is shown to shift the monomers that are involved in crossover points and produce the different patterns in crossover periods which have been observed from electron micrographs analysis. We specifically demonstrate that for a constant nonzero rotational offset the length of the crossover periods alternates, whereas for a constant offset of zero the crossover period remains constant. We also show that changes in the rotational offset angle along the filament can account for variable (random) crossover periods.