In anaphase, microtubules provide a specification signal for positioning of the contractile ring. However, the nature of the signal remains unknown. The small GTPase Rho is a potent regulator of cytokinesis, but the involvement of Rho in contractile ring formation is disputed. Here, we show that Rho serves as a microtubule-dependent signal that specifies the position of the contractile ring. We found that Rho translocates to the equatorial region before furrow ingression. The Rho-specific inhibitor C3 exoenzyme and small interfering RNA to the Rho GDP/GTP exchange factor ECT2 prevent this translocation and disrupt contractile ring formation, indicating that active Rho is required for contractile ring formation. ECT2 forms a complex with the GTPase-activating protein MgcRacGAP and the kinesinlike protein MKLP1 at the central spindle, and the localization of ECT2 at the central spindle depends on MgcRacGAP and MKLP1. In addition, we show that the bundled microtubules direct Rho-mediated signaling molecules to the furrowing site and regulate furrow formation. Our study provides strong evidence for the requirement of Rho-mediated signaling in contractile ring formation.

f Cytokinesis is a crucial step in the creation of two daughter cells by the formation and ingression of the cleavage furrow. Here, we show that sphingomyelin (SM), one of the major sphingolipids in mammalian cells, is required for the localization of phosphatidylinositol-4,5-bisphosphate (PIP 2 ) to the cleavage furrow during cytokinesis. Real-time observation with a labeled SMspecific protein, lysenin, revealed that SM is concentrated in the outer leaflet of the furrow at the time of cytokinesis. Superresolution fluorescence microscopy analysis indicates a transbilayer colocalization between the SM-rich domains in the outer leaflet and PIP 2 -rich domains in the inner leaflet of the plasma membrane. The depletion of SM disperses PIP 2 and inhibits the recruitment of the small GTPase RhoA to the cleavage furrow, leading to abnormal cytokinesis. These results suggest that the formation of SM-rich domains is required for the accumulation of PIP 2 to the cleavage furrow, which is a prerequisite for the proper translocation of RhoA and the progression of cytokinesis.

The behavior of sphingomyelin (SM) in the plasma membrane is unclear. Using new fluorescent SM analogs and single-molecule tracking, Kinoshita et al. find that SM transiently and continually interacts with the glycosylphosphatidylinositol (GPI)-anchored protein CD59 in oligomer size–, cholesterol-, and GPI anchoring–dependent manners.

Sphingomyelin (SM) is a major sphingolipid in mammalian cells and is reported to form specific lipid domains together with cholesterol. However, methods to examine the membrane distribution of SM are limited. We demonstrated in model membranes that fluorescent protein conjugates of 2 specific SM-binding toxins, lysenin (Lys) and equinatoxin II (EqtII), recognize different membrane distributions of SM; Lys exclusively binds clustered SM, whereas EqtII preferentially binds dispersed SM. Freeze-fracture immunoelectron microscopy showed that clustered but not dispersed SM formed lipid domains on the cell surface. Glycolipids and the membrane concentration of SM affect the SM distribution pattern on the plasma membrane. Using derivatives of Lys and EqtII as SM distribution-sensitive probes, we revealed the exclusive accumulation of SM clusters in the midbody at the time of cytokinesis. Interestingly, apical membranes of differentiated epithelial cells exhibited dispersed SM distribution, whereas SM was clustered in basolateral membranes. Clustered but not dispersed SM was absent from the cell surface of acid sphingomyelinasedeficient Niemann-Pick type A cells. These data suggest that both the SM content and membrane distribution are crucial for pathophysiological events bringing therapeutic perspective in the role of SM membrane distribution.-Makino, A., Abe, M., Murate, M., Inaba, T.,