The Schizosaccharomyces pombe Cdc42p GTPase signals through Pak2p and the Mkh1p-Pek1p-Spm1p MAP kinase pathway

The Cdc42p GTPase is involved in many aspects of growth and cell-cycle regu lation, including actin cytoskeletal rearrangements and activation of signa l transduction pathways. To further investigate these functions, genetic in teractions were examined between Schizosaccharomyces pombe Cdc42p, its effe cters Pak1p and Pak2p, and the Mkh1p-Pek1p-Spm1p signal transduction pathwa y, which functions in cytokinesis and cell division. Expression of a trunca ted version of Pak2p lacking its N-terminal autoinhibitory domain led to a growth defect that was suppressed by Delta mkh1 and Delta spm1 null mutatio ns and an elongated cell phenotype indicative of a cell division defect tha t was suppressed by the Delta mkh1 mutation. In addition, expression of the constitutively activated cdc42(G12V) mutant allele led to a growth defect that was rescued by the Delta pak2 and Delta mkh1 mutations. The Delta pak2 mutation did not suppress the growth defect conferred by plasmid expressio n of Mkh1p, suggesting that Pak1p functions upstream of Mkh1p in this pathw ay. A two-hybrid protein interaction was observed between Pak2p and Mkh1p, but not between Pak1p and Mkh1p. These results are consistent with Cdc42p i nteracting with Pak2p to signal through the Mkh1p-Pek1p-Spm1p pathway.