The Cdc42p GTPase is targeted to the site of cell division in the fission yeast Schizosaccharomyces pombe

The Rho-family GTPase Cdc42p regulates many aspects of cell polarity and gr owth in eukaryotic cells, including the organization of the actin cytoskele ton. To further examine Cdc42p function in the fission yeast Schizosaccharo myces pombe, a functional green fluorescent protein (GFP)-Cdc42p fusion pro tein was generated. GFP-Cdc42p was observed at the medial region of the cel l at the cell-division site early in cytokinesis and remained there through cell separation, and was also localized to the periphery of the cell and t o internal membranes. Unexpectedly, treatment with the actin-depolymerizing drug latrunculin-A disrupted the medial region targeting pattern, and cell s deficient in the actin-binding proteins tropomyosin and profilin also did not exhibit medial GFP-Cdc42p staining. In addition, medial GFP-Cdc42p loc alization was eliminated in a number of cytokinesis mutants, including stra ins defective in assembling the medial actinomyosin ring, medial ring contr action, and septum assembly. GFP-Cdc42p targeting was less affected in muta nts that formed misplaced or multiple septa. These results suggest that the localization of Cdc42p at the cell-division site was dependent upon the ac tin cytoskeleton and that Cdc42p may function in the interdependent process es of cytokinesis and septation.