ROLE OF THE YEAST GIN4P PROTEIN-KINASE IN SEPTIN ASSEMBLY AND THE RELATIONSHIP BETWEEN SEPTIN ASSEMBLY AND SEPTIN FUNCTION

To identify septin-interacting proteins in Saccharomyces cerevisiae, w e screened for mutations that are synthetically lethal with a cdc12 se ptin mutation. One of the genes identified was GIN4, which encodes a p rotein kinase related to Hsl1p/Nik1p and Ycl024Wp in S. cerevisiae and to Nim1p/Cdr1p and Cdr2p in Schizosaccharomyces pombe. The Gin4p kina se domain displayed a two-hybrid interaction with the COOH-terminal po rtion of the Cdc3p septin, and Gin4p colocalized with the septins at t he mother-bud neck. This localization depended on the septins and on t he COOH-terminal (nonkinase) region of Gin4p, and overproduction of th is COOH-terminal region led to a loss of septin organization and assoc iated morphogenetic defects. We detected no effect of deleting YCL024W , either alone or in combination with deletion of GIN4. Deletion of GI N4 was not lethal but led to a striking reorganization of the septins accompanied by morphogenetic abnormalities and a defect in cell separa tion; however, remarkably, cytokinesis appeared to occur efficiently. Two other proteins that localize to the neck in a septin-dependent man ner showed similar reorganizations and also appeared to remain largely functional. The septin organization observed in gin4 Delta vegetative cells resembles that seen normally in cells responding to mating pher omone, and no Gin4p was detected in association with the septins in su ch cells. The organization of the septins observed in gin4 Delta cells and in cells responding to pheromone appears to support some aspects of the model for septin organization suggested previously by Field et al. (Field, C.M., O. Al-Awar, J. Rosenblatt, M.L. Wong, B. Alberts, an d T.J. Mitchison. 1996. J. Cell Biol. 133:605-616).