Evolutionary conservation of Xenopus laevis mitogen-activated protein kinase activation and function

Saccharomyces cerevisiae possesses at least four mitogen-activated protein (MAP) kinase family members, encoded by the FUS3, KSS1, HOG1, and MPK1 genes, that participate in three distinct signaling pathways. We have tested whether a MAP kinase from Xenopus laevis (Xp42) can function in budding yeast, by expressing wild-type and mutant forms of Xp42 in different strains of S. cerevisiae. In Xenopus cells, Xp42 is phosphorylated on threonine188 and tyrosine190 when activated by a MAP kinase kinase (MAPKK). In S. cerevisiae, Xp42 is constitutively phosphorylated on tyrosine190. Since a kinase-inactive mutant of Xp42 is also phosphorylated, this phosphorylation is presumably due to activation by an endogenous MAPKK. Xp42 phosphorylation and kinase activity are dependent on yeast Bck1p, a putative MAPKK kinase (MAPKKK) and indirect upstream activator of Mpk1p. The loss of either Ste7p or Pbs2p, the upstream activators of Fus3p, Kss1p, and Hog1p, does not decrease the phosphorylation stoichiometry of Xp42. We also show that expression of Xenopus MAP kinase permits an mpk1::TRP1 deletion strain to grow at 37 degrees C. We conclude that S. cerevisiae and X. laevis possess evolutionarily conserved cascades, where biochemical activation and substrate specificity of MAP kinase have been maintained.