hypothesized that anillin couples the actomyosin cytoskeleton to microtubules by binding MgcRacGAP/CYK-4/RacGAP50C. Anillins have also been shown to organize the actomyosin cytoskeleton into syncytial structures observed in Drosophila embryos or C. elegans gonads. ANI-1 and ANI-2 (proteins homologous to anillin) are essential for embryonic viability in both organisms. ANI-1 is required for cortical ruffling, pseudocleavage, and all contractile events that occur in embryos prior to mitosis. ANI-1 is also crucial for segregation of polar bodies during meiosis. ANI-2 functions in the maintenance of the structure of the central core of the cytoplasm, the rachis, during oogenesis. ANI-2 ensures oocytes do not disconnect prematurely from the rachis, thereby leading to the generation of embryos of varying sizes. In vitro experiments suggest that anillin drives myosin-independent actin contractility. Binding Partners Actin Anillin specifically binds F-actin, rather than G-actin. Binding of F-actin by anillin only occurs during cell division. Anillin also bundles actin filaments together and drives their relative sliding. This contractile behavior is independent of myosin and ATP and may couple with actin filament disassembly. Amino acids 258-340 are sufficient and necessary for F-actin binding in Drosophila, but amino acids 246-371 are necessary to bundle actin filaments. The ability of anillin to bind to and bundle actin together is conversed through many species. It is hypothesized that by regulating actin bundling, anillin increases the efficiency of actomyosin contractility during cell division. Both anillin and F-actin are found in contractile structures. They are recruited independently to the contractile ring, but F-actin increases the efficiency of anillin targeting. Anillin may also be involved in promoting the polymerization of F-actin by stabilizing formin mDia2 in an active form. Myosin Anillin interacts directly with non-muscle myosin II and interacts indirectly with myosin via F-actin. Residues 142-254 (near the N-terminus) are essential for anillin binding myosin in Xenopus. The interaction of anillin and myosin is also dependent on phosphorylation of the myosin light chain. The interaction of myosin and anillin does not seem to serve in recruitment, but rather organization of myosin. In Drosophila, anillin is necessary to organize myosin into rings in the cellularization front. Depletion of anillin in Drosophila and humans leads to changes in the spatial and temporal stability of myosin during cytokinesis. In C. elegans, ANI-1 organizes myosin into foci during cytokinesis and establishment of polarity, whereas, ANI-2 is a requirement for the maintenance of myosin-rich contractile lining of oogenic gonads. Septins Septin