The actin-based motor myosin Va is a component of the acroplaxome, an acrosome-nuclear envelope junctional plate, and of manchette-associated vesicles.

Protein and vesicle cargos can be mobilized during spermiogenesis by intramanchette transport utilizing microtubule-based protein motors (kinesins and dyneins). However, actin-based unconventional myosin motors may also play a significant role in targeting vesicle cargos to subcellular compartments during sperm development. Here we report that myosin Va, an actin-based motor protein, is a component of the acroplaxome of rodent spermatids. The acroplaxome is an F-actin/keratin-containing scaffold plate with a marginal ring fastening the caudal recess of the developing acrosome to the nuclear envelope during spermatid nuclear shaping. In contrast to the acroplaxome, fluorescently labeled phalloidin does not produce an obvious F-actin signal in the manchette. However, immunogold electron microscopy detects moderate but specific beta-actin immunoreactivity along interconnected tube-like bundles of manchette microtubules. We also show that the membrane of vesicles co-fractionated with intact manchettes by sucrose gradient ultracentrifugation display immunogold-labeled myosin Va. Myosin Va vesicle localization is known to correlate with Rab proteins, monomeric GTPases of the Ras superfamily which recruit myosin Va/VIIa motor proteins through intermediate proteins. RT-PCR analysis demonstrates that transcripts for Rab27a and Rab27b and Slac2-c (a protein that links Rab27a/b to myosin Va/VIIa) are expressed in testis. These results indicate that two independent cytoskeletal tracks, F-actin in the acroplaxome and presumably in the manchette, and manchette microtubules, may facilitate short-range (from the Golgi to the acrosome) and long-range (from the manchette to the centrosome and axoneme) mobilization of appropriate cargos during spermiogenesis. Copyright 2003 S. Karger AG, Basel