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Recently, a new family of proteins, termed IQGAP, have been implicated in connecting Cdc42 and calmodulin signaling to the remodeling of the actin cytoskeleton ( Weissbach et al., 1994 Brill et al., 1996 Hart et al., 1996 McCallum et al., 1996 Bashour et al., 1997). The direct targets of the small GTP-binding proteins and calmodulin in cytokinesis have not yet been identified. Calcium/calmodulin-dependent signaling events are also thought to regulate the onset and progression of actin ring contraction (for review see Schroeder, 1990 Walsh, 1994). Mutational or pharmacological inactivation of Rho-type GTPases, such as Rho and Cdc42, inhibits cell cleavage ( Kishi et al., 1993 Drechsel et al., 1996 Larochelle et al., 1996). Proteins known to regulate actin cytoskeleton organization are likely to play important roles in contractile ring formation and function. How actin and myosin are recruited to the cleavage site to form the contractile structure is not well understood. The accumulation of actin in the cytokinetic ring was not observed in cells depleted of Cyk1p, suggesting that Cyk1p plays a role in the recruitment of actin filaments, perhaps through a filament-binding activity similar to that demonstrated for mammalian IQGAPs.Ĭ ytokinesis in animal cells depends on drastic rearrangements of cortical actin filaments, leading to the assembly of an actomyosin-based contractile ring (for review see Schroeder, 1990 Satterwhite and Pollard, 1992 Fishkind and Wang, 1995). The localization of myosin is abolished by a mutation in Cdc12p, implicating a role for the septin filaments in the assembly of the actomyosin ring.

The assembly of this ring occurs in temporally distinct steps: Myo1p localizes to a ring that overlaps the septins at the G1-S transition slightly before bud emergence Cyk1p and actin then accumulate in this ring after the activation of the Cdc15 pathway late in mitosis. These results indicate that cytokinesis in budding yeast is likely to involve an actomyosin-based contractile ring. In vivo observation with green fluorescent protein–tagged Myo1p showed that the ring decreases drastically in size during cell division and therefore may be contractile. This ring contains filamentous actin and Myo1p, a myosin II homologue. Cyk1p is diffused throughout most of the cell cycle but localizes to a ring structure at the mother–bud junction after the initiation of anaphase. Gene disruption of Cyk1p results in a failure in cytokinesis without affecting other events in the cell cycle. We have identified a Saccharomyces cerevisiae protein, Cyk1p, that exhibits sequence similarity to the mammalian IQGAPs.