The regulatory phosphorylation sites of MAP kinases. Xenopus and mammalian MAP kinases become active only when both threonine and tyrosine residues are phosphorylated. These regulatory phosphofflation sites are on the TEY sequence (tinted) located just before the APE sequence of the kinase subdomain Vii;(bracketed). The sequence between TEY and APE is completely conserved in yeast homologs of MAP kinase such as FUS3, KSS1, MPK1 and spkl (see text for details). to Byrl, Wisl, PSB2 and STET, respectively, over their shared lengths; within the kinase domain the identity is higher (53%, 52%, 52% and 43%, respectively). Bacterially expressed recombinant Xenopus MAPKK was recognized by anti-MAPKK antibody and was capable of undergoing weak autophosphorylation on serine, threonine and tyrosine residues. Further, recombinant MAPKK was able to phosphorylate and activate recombinant MAPK, after activation by incubation with Xenopus egg extracts 25. These results are consistent with previous studies show! ng that activated MAPKK alone is sufficient for MAPK activation and that MAPKK is a protein klnase with dual specificity.

Recent genetic and biochemical work on S. cerevisiae s has clearly shown that STE7, one of the MAPKK homologs, lies immediately upstream of the MAPK homologs, FUS3/KSSI, in the mating signalling pathway. Thus, we can hypothesise that other yeasts MAPKs and putative MAPKKs might also interact directly in other signalling pathways (Fig. 3). For example, Byrl (Ref. 30) is supposed to function downstream of Rasl, an S. pombe homolog of vertebrate Ras p21 and is essential to the sexual differentiation pathway. Since Spkl (Ref. 31), a MAPK homolog, is also essential to the same signaling pathway, Byrl could be an immediate upstream of Spkl. As we described earlier,