The Plant Genome Encodes a Variety of MAPKs MAPKKs and MAPKKKs

Members of each of the MAPK, MAPKK and MAPKKK families have been identified by genome projects using various model plants. In the Arabidop-sis genome, 20 MAPKs, ten MAPKKs and 60 MAPKKKs were found (MAPK Group, 2002). In addition to the Arabidopsis genome, the rice (Oryza sativa) and poplar (Populus trichocarpa) genomes have been sequenced and shown to encode 15 and 21 MAPKs, and eight and 11 MAPKKs, respectively (Hamel et al. 2006). Thus, plants possess more MAPKs and MAPKKs than yeast (six for each) and the human (ten MAPKs and seven MAPKKs), indicating the complexity of the MAP kinase pathways in plants. Since the genomes of the three model plants described above encode fewer MAPKKs than MAPKs, a single MAPKK might activate multiple MAPKs. However, very few relationships between combinations of plant MAPKKs and MAPKs have been established.

Although more than 60 genes in the Arabidopsis genome are predicted to encode MAPKKKs, some of them may not function as MAPKKKs, such as MAP3Ke1, whose homolog, CDC7 in Schizosaccharomyces pombe and CDC15 in Saccharomyces cerevisiae, have been classified into a different family of protein kinases (Jouannic et al. 2001; Champion et al. 2004). Based on the conserved similarity of the kinase domain, more than 60 predicted proteins in Arabidopsis are classified as members of the MAPKKK family, but the complete primary structures of some members are far from the typical MAPKKK structure, which makes it difficult to classify all defined MAPKKK candidates in a single protein family (MAPK Group 2002). It seems necessary to reclassify these divergent members into suitable protein families. Although the precise number of MAPKKKs in Arabidopsis is still to be established, this plant apparently possesses a number of protein kinases that are structurally similar to MAPKKK.

0 0

Post a comment