Image analysis shows that variations in actin crossover spacings are random, not compensatory.

A recent paper by Bremer et al. (1991. J. Cell Biol. 115:689-703) has argued that the random angular disorder model for actin is wrong, and that the variations in crossover spacing observed in electron micrographs of F-actin filaments can be best explained by a compensatory disorder caused by the lateral slipping of the twin (or two-start) strands which comprise the actin filament. We have analyzed the images of F-actin presented in Bremer et al. and show that their data argues against compensatory disorder and in favor of random disorder, independent of the cause of the disorder. We also revise our estimate of the angular component and show that the magnitude of this disorder is about 5-6 degrees per subunit, which is less than the 10-12 degrees that we originally proposed.