Myosin VI Stabilizes an Actin Network duringDrosophilaSpermatid Individualization

Abstract

Here, we demonstrate a new function of myosin VI using observations of Drosophila spermatid individualization in vivo. We find that myosin VI stabilizes a branched actin network in actin structures (cones) that mediate the separation of the syncytial spermatids. In a myosin VI mutant, the cones do not accumulate F-actin during cone movement, whereas overexpression of myosin VI leads to bigger cones with more F-actin. Myosin subfragment 1-fragment decoration demonstrated that the actin cone is made up of two regions: a dense meshwork at the front and parallel bundles at the rear. The majority of the actin filaments were oriented with their pointed ends facing in the direction of cone movement. Our data also demonstrate that myosin VI binds to the cone front using its motor domain. Fluorescence recovery after photobleach experiments using green fluorescent protein-myosin VI revealed that myosin VI remains bound to F-actin for minutes, suggesting its role is tethering, rather than transporting cargo. We hypothesize that myosin VI protects the actin cone structure either by cross-linking actin filaments or anchoring regulatory molecules at the cone front. These observations uncover a novel mechanism mediated by myosin VI for stabilizing long-lived actin structures in cells.

The testis-specific proteasome subunit Prosα6T of D. melanogaster is required for individualization and nuclear maturation during spermatogenesis

“…Although the mechanism of the actin cone movement has not been fully elucidated, it has been proposed that actin polymerization and depolymerization are the driving force (Noguchi and Miller, 2003). Myosin VI seems to be an actin cone structure stabilizer (Noguchi et al, 2006), and myosin V is required for actin cone formation (Mermall et al, 2005). Here, we demonstrate that the ubiquitin-proteasome pathway is also important in IC movement.…”

OSBP- and FAN-mediated sterol requirement for spermatogenesis inDrosophila

“…Individualization is a membrane reorganizing process during which an actin-based complex, the individualization complex (IC), composed of a cohort of 64 actin cones, progresses caudally along the spermatid heads and remodels the syncytial membrane to remove excess cytosol and pack each spermatid into its own plasma membrane (Fuller, 1993). Mutants that compromise the caspase pathway, the integrity or the movement of the IC, or the trafficking of intracellular vesicles have been shown to affect the individualization process (Fabrizio et al, 1998;Huh et al, 2004;Noguchi et al, 2006;Noguchi and Miller, 2003;Sevrioukov et al, 2005). We found that OSBP-positive and sterol-rich speckles are present at the leading edge of the IC, suggesting that they might participate in the membrane remodelling process.…”

yuri gagarinis required for actin, tubulin and basal body functions inDrosophilaspermatogenesis

“…The formation of the F-actin cones of individualization has been studied previously (Fabrizio et al, 1998;Lindsley and Tokuyasu, 1980; Noguchi et al, 2006). Initially, actin fibers accrete along the lengths of the condensed sperm nuclei in the basal testis.…”

“…Yuri is also a component of the actin cones that mediate sperm individualization and is required for their formation. The actin cones are formed by a two-step process (Noguchi et al, 2006). Initially, parallel actin fibers are formed around the nuclei and then an actin meshwork is added at each apical nuclear tip.…”