Structure of actin filament bundles from microvilli of sea urchin eggs

The detailed substructure of actin filament bundles in microvilli of fertilized sea urchin eggs has been studied by analysing electron microscope images of negatively stained specimens. Transverse stripes which repeat about every 130 Å along the axis of a bundle, as previously observed by Burgess & Schroeder (1977), reflect the positions of cross-bridges that connect the filaments into a bundle. Analysis of optical transforms of the micrographs reveals that there are approximately 14 actin monomers between cross-overs of the two long-pitch helical strands of the actin filaments, with three cross-bridges in this interval. The structure is basically similar to that of the hexagonally packed bundles prepared in vitro from high speed supernatants of sea urchin eggs by Kane (1975) and analyzed by DeRosier et al. (1977). One clear difference, however, is that the in vivo microvillar filament bundles are supercoiled, giving rise to long axial repeats of 1500 to 2000 Å.Computationally filtered images of regions that were only slightly supercoiled reveal the relative alignment of filaments within the bundles and show that crossbridges appear to interact with four actin monomers, apparently linking two actin monomers on one strand of one filament to the nearest two monomers on a neighbouring filament. However, the cross-bridges are not spaced at equal intervals corresponding to four actin subunits, presumably because of the lack of hexagonal symmetry in the individual filaments, which have about 14 actin monomers between cross-overs. Instead, the cross-bridges are arranged quasiequivalently along the longitudinal axis of the bundles, in steps of four or five actin subunit spacings (28 Å each).