A PATHWAY IN THE YEAST-CELL DIVISION CYCLE LINKING PROTEIN-KINASE-C (PKC1) TO ACTIVATION OF CDC28 AT START

In an effort to study further the mechanism of Cdc28 function and cell cycle commitment, we describe here a genetic approach to identify com ponents of pathways downstream of the Cdc28 kinase at START by screeni ng for mutations that decrease the effectiveness of signaling by Cdc28 . The first locus to be characterized in detail using this approach wa s PKC1 which encodes a homolog of the Ca2+-dependent isozymes of the m ammalian protein kinase C (PKC) superfamily (Levin et al., 1990), By s everal genetic criteria, we show a functional interaction between CDC2 8 and PKC1 with PKC1 apparently functioning with respect to bud emerge nce downstream of START, Consistent with this, activity of the MAP kin ase homolog Mpk1 (a putative Pkc1 effector) is stimulated by activatio n of Cdc28. Furthermore, we demonstrate a cell cycle-dependent hydroly sis of phosphatidylcholine to diacylglycerol (a PKC activator) and cho line phosphate at START, Diacylglycerol production is stimulated by Cd c28 in cycling cells and is closely associated with Cdc28 activation a t START. These results imply that the activation of Pkc1, which is kno wn to be necessary during bud morphogenesis, is mediated via the CDC28 -dependent stimulation of PC-PLC activity in a novel cell cycle-regula ted signaling pathway.