Abstract
DeRosier et al. (1977) have proposed a structural model for the needles, which are bundles of hexagonally packed actin filaments crosslinked by fascin, a 58,000 Mr protein. An unusual consequence of their model is that the crossbridges are not uniformly spaced but follow a distinctive pattern whose repeat depends on the symmetry of the actin. From electron micrographs of needles, we have determined both the symmetry of the actin and the repeat of the pattern of crossbridges using a novel image-filtering algorithm. We find the pattern of crossbridges is correctly predicted by the symmetry of the actin. As part of the analysis, we produce a one-dimensional filtered image of the crossbridge which consists of a bilobal peak about 60 Å long in its axial dimension. More interestingly, the repeat of the pattern of crossbridges is so sensitive to the symmetry of the filaments that it allows us to determine actin symmetry to one part in 20,000 (i.e. the actin filaments in Fig. 1 have 2.1590 ± 0.0001 units per turn). Finally the filtering algorithm allows us to determine unambiguously the polarity of the bundle.
The algorithm we developed is an extension of computerized image filtering that is applicable to structures such as the needle in which the translational intervals between repeated objects are not all the same.