Complementation of byrl in fission yeast by mammalian MAP kinase kinase requires coexpression of Raf kinase

Abstract

INTRACELLULAR signalling from receptor tyrosine kinases in mammalian cells involves the activation of a signal cascade which includes p21^ras and the protein kinases p74^raf-1, MAP kinase kinase and MAP kinases^1–8. In the yeasts Schizosaccharomyces pombe and Saccharomyces cerevisiae the response to mating pheromones requires the Spkl and KSS1/FUS3 kinases, which have sequence homology to vertebrate MAP kinases^9–12. The recent cloning of complementary DNAs for mammalian^13–15 and frog¹⁶ MAP kinase kinases has shown that they are homologous to the S. pombe Byr1 (ref. 17) and S. cerevisiae STE7 (ref. 18) kinases, which have been proposed to function upstream of Spk1 and KSS1/FUS3, respectively^19–22. We have investigated whether these apparently similar kinase pathways are functionally conserved between vertebrates and S. pombe. We report here that expression of mammalian MAP kinase kinase alone fails to complement a byr1 mutant of S. pombe. When coexpressed with Raf kinase, however, MAP kinase kinase is activated by phosphorylation and the mating defect of the byr1 mutant is rescued. This suggests that the pathways are functionally homologous and that Raf kinase may directly phosphorylate and activate MAP kinase kinase.