Polymerization of actin. VI. The polarity of the actin filaments in the acrosomal process and how it might be determined.

Abstract

The polarity of the actin filaments which assemble from the nucleating body or actomere of Thyone briareus and Pisaster ochraceus sperm was determined using myosin subfragment 1 decoration. The polarity was unidirectional with the arrowheads pointing towards the cell center. When polymerization is induced at low temperature with concentrations of actin near the critical concentration for polymerization, elongation of filaments occurs preferentially off the apical end. If the sperm are induced to undergo the acrosomal reaction with an ionophore, the polarity of the actin filaments attached to the actomere is the same as that already described, but the filaments which polymerize parallel to, but peripheral to, those extending from the actomere are randomly polarized. These randomly polarized filaments apparently result from spontaneous nucleation. When sperm are induced to undergo the acrosomal reaction with eggs, the polarity of the actin filaments is also unidirectional with the arrowheads pointing towards the cell center. The actomere, by nucleating the polymerization of actin filaments, controls the polarity of the actin filaments in the acrosomal process. The actomere recognizes a surface of the actin monomer that is different from that surface recognized by the dense material attached to membranes. Egg myosin could not act to pull the sperm into the egg. Included is a discussion of how the observation that monomers add largely to 1 end of a decorated filament in vitro relates to these in vivo observations.