Plant MAP kinase kinase kinases structure, classification and evolution

The increasing number of reports describing plant MAP kinase signalling com ponents reflects the cardinal role that MAP kinase pathways are likely to p lay during plant growth and development. Relationship and structural analys es of plant MAP kinase kinase kinase related cDNAs and genes established, o n one hand, the PMEKKs, which may be distinguished into the alpha, beta, ga mma, and zeta groups, and, on the other hand, the PRAFs that consist of the delta, eta and theta groups. Plant MAP3Ks are characterized by different p rimary structures, but conserved within a single group. A relationship anal ysis, which included animal, fungal and plant MAP3Ks, revealed a high degre e of diversity among this biochemically established set of proteins, thus s uggesting a range of biological functions. Four major families emerged, nam ely the MEKK/STE11, including the PMEKKs, the RAF, including the PRAFs, as well as the MLK and CDC7 families. These four families showed phylum-depend ent distributions. Signature sequences characterizing the RAF family and th e RAF subfamilies have been evidenced. However, no equivalent sequence moti fs were identified for the MEKK/STE11 family, which is highly heterogeneous . (C) 1999 Elsevier Science B.V. All rights reserved.