Tatsuhiko Noguchi

Phosphatidylinositol 4,5-bisphosphate Directs Spermatid Cell Polarity and Exocyst Localization in Drosophila

This study identifies phosphoinositides as key regulators of spermatid cell polarity. Polarization and elongation of spermatids in Drosophila are regulated through local synthesis of PIP2 by Sktl, which drives polarized localization of the exocyst complex to promote targeted membrane delivery and polarization of the elongating spermatid cysts.

Proper Cellular Reorganization during Drosophila spermatid Individualization Depends on Actin Structures Composed of Two Domains, Bundles and Meshwork, that Are Differentially Regulated and Have Different Functions

During spermatid individualization in Drosophila, actin structures (cones) mediate cellular remodeling that separates the syncytial spermatids into individual cells. These actin cones are composed of two structural domains, a front meshwork and a rear region of parallel bundles. We show here that the two domains form separately in time, are regulated by different sets of actin-associated proteins, can be formed independently, and have different roles. Newly forming cones were composed only of bundles, whereas the meshwork formed later, coincident with the onset of cone movement. Polarized distributions of myosin VI, Arp2/3 complex, and the actin-bundling proteins, singed (fascin) and quail (villin), occurred when movement initiated. When the Arp2/3 complex was absent, meshwork formation was compromised, but surprisingly, the cones still moved. Despite the fact that the cones moved, membrane reorganization and cytoplasmic exclusion were abnormal and individualization failed. In contrast, when profilin, a regulator of actin assembly, was absent, bundle formation was greatly reduced. The meshwork still formed, but no movement occurred. Analysis of this actin structures formation and participation in cellular reorganization provides insight into how the mechanisms used in cell motility are modified to mediate motile processes within specialized cells.

Coiled-Coil–Mediated Dimerization Is Not Required for Myosin VI to Stabilize Actin during Spermatid Individualization in Drosophila melanogaster

Myosin VI is a pointed-end-directed actin motor that is thought to function as both a transporter of cargoes and an anchor, capable of binding cellular components to actin for long periods. Dimerization via a predicted coiled coil was hypothesized to regulate activity and motor properties. However, the importance of the coiled-coil sequence has not been tested in vivo. We used myosin VIs well-defined role in actin stabilization during Drosophila spermatid individualization to test the importance in vivo of the predicted coiled coil. If myosin VI functions as a dimer, a forced dimer should fully rescue myosin VI loss of function defects, including actin stabilization, actin cone movement, and cytoplasmic exclusion by the cones. Conversely, a molecule lacking the coiled coil should not rescue at all. Surprisingly, neither prediction was correct, because each rescued partially and the molecule lacking the coiled coil functioned better than the forced dimer. In extracts, no cross-linking into higher molecular weight forms indicative of dimerization was observed. In addition, a sequence required for altering nucleotide kinetics to make myosin VI dimers processive is not required for myosin VIs actin stabilization function. We conclude that myosin VI does not need to dimerize via the predicted coiled coil to stabilize actin in vivo.