Elongation of cytoplasmic processes during gametic mating: models for actin-based motility.

The acrosomal processes of Thyone and Limulus sperm and the fertilization tubule of mt+ gametes of Chlamydomonas are interesting models for actin-based motility. Each is a long thin process that elongates rapidly and contains a core of actin filaments having uniform polarity: arrowheads formed by myosin subfragments point toward the base of the processes. In Limulus, directed outgrowth of the acrosomal process is achieved by a rearrangement in the packing of superhelically coiled actin filaments that form during spermatogenesis. In contrast, outgrowth of the acrosomal process in Thyone and the fertilization tubule in Chlamydomonas is accompanied by actin polymerization. Both Thyone and Chlamydomonas possess structures, the actomere and the doublet zone, respectively, that serve as microfilament organizing centers, nucleating actin polymerization and defining the polarity of the growing filaments. Alkalinization of the cytoplasm may promote polymerization of actin in Thyone, whereas an apparent rise in the intracellular Ca2+ concentration is associated with the transmission of intracellular signals during mating in Chlamydomonas. Further examination of these three actin-based motile systems should provide new insights into the assembly of the actin cytoskeleton, a process critical for many forms of nonmuscle cellular motility.