THE SLT2(MPK1) MAP KINASE IS ACTIVATED DURING PERIODS OF POLARIZED CELL-GROWTH IN YEAST

The SLT2(MPK1) mitogen-activated protein kinase signal transduction pa thway has been implicated in several biological processes in Saccharom yces cerevisiae, including the regulation of cytoskeletal and cell wal l structure, polarized cell growth, and response to nutrient availabil ity, hypo-osmotic shock and heat shock, We examined the conditions und er which the SLT2 pathway is activated, We found that the SLT2 kinase is tyrosine phosphorylated and activated during periods in which yeast cells are undergoing polarized cell growth, namely during bud formati on of vegetative cell division and during projection formation upon tr eatment with mating pheromone, BCK1(SLK1), a MEK kinase, is required f or SLT2 activation in both of these situations, Upstream of BCK1(SLK1) , we found that the STE20 kinase was required for SLT2 activation by m ating pheromone, but was unnecessary for its activation during the veg etative cell cycle, Finally, SLT2 activation during vegetative growth was partially dependent on CDC28 in that the stimulation of SLT2 tyros ine phosphorylation was significantly reduced directly after a tempera ture shift in cdc28 ts mutants, Our data are consistent with a role fo r SLT2 in promoting polarized cell growth.