The polymerization of actin: II. how nonfilamentous actin becomes nonrandomly distributed in sperm: evidence for the association of this actin with membranes

Abstract

At an early stage in spermiogenesis [in Thyone briareus] the acrosomal vacuole and other organelles including ribosomes are located at the basal end of the cell. From here actin must be transported to its future location at the anterior end of the cell. At no stage in the accumulation of actin in the periacrosomal region is the actin sequestered in a membrane-bounded compartment such as a vacuole or vesicle. Since filaments are not present in the periacrosomal region during the accumulation of the actin even though the fixation of these cells is sufficiently good to distinguish actin filaments in thin section, the actin must accumulate in the nonfilamentous state. The membranes in the periacrosomal region, specifically a portion of the nuclear envelope and the basal half of the acrosomal vacuole membrane, become specialized morphologically in advance of the accumulation of actin in this region. The working hypothesis was that actin in combination with other substances binds to these specialized membranes and to itself and thus accumulates in the periacrosomal region by being trapped on these specialized membranes. Diffusion would then be sufficient to move these substances to this region. Mature sperm were treated with detergents, glycols and hypotonic media which solubilize or lift away the plasma membrane. The actin and its associated proteins remained attached to these specialized membranes. Actin can be nonrandomly distributed in cells in a nonfilamentous state presumably by its association with specialized membranes.