Cdc4p, a contractile ring protein essential for cytokinesis in Schizosaccharomyces pombe, interacts with a phosphatidylinositol 4-kinase

The proposed function of Cdc4p, an essential contractile ring protein in Sc hizosaccharomyces pombe, is that of a myosin essential light chain. However , five conditionally lethal cdc4 alleles exhibit complementation in diploid s. Such interallelic complementation is not readily explained if the sole f unction of Cde4p is that of a myosin essential light chain. Complementation of cdc4 alleles could occur only if different mutant forms can assemble in to an active oligomeric complex or if Cdc4p has more than one essential fun ction. To search for other proteins that may interact with Cdc4p, we perfor med a two-hybrid screen and identified two such candidates: one similar to Saccharomyces cerevisiae Vps27p and the other a putative phosphatidylinosit ol (PI) 4-kinase. finding of Cdc4p to the latter and to myosin heavy chain (Myo2p) was confirmed by immunosorbent assays. Deletion studies demonstrate d interaction between the Cdc4p C-terminal domain and the PI 4-kinase C-ter minal domain. Furthermore, interaction was abolished by the Cdc4p C-termina l domain point mutation, Gly(107) to Ser. This allele also causes failure o f cytokinesis. Ectopic expression of the PI 4-kinase C-terminal domain caus ed cytokinesis defects that were most extreme in cells carrying the G107S a llele. We suggest that Cdc4p plays multiple roles in cytokinesis and that i nteraction with a PI I-kinase may be important for contractile ring assembl y and/or function.