Catalytic domain of Plant dual-specificity Mitogen-Activated Protein Kinase Kinases and similar proteins
PKs catalyze the transfer of the gamma-phosphoryl group from ATP to serine/threonine (ST) or tyrosine residues on protein substrates. Members of this group include MAPKKs from plants, kinetoplastids, alveolates, and mycetozoa. The MAPKK, LmxPK4, from Leishmania mexicana, is important in differentiation and virulence. Dictyostelium discoideum MEK1 is required for proper chemotaxis; MEK1 null mutants display severe defects in cell polarization and directional movement. Plants contain multiple MAPKKs like other eukaryotes. The Arabidopsis genome encodes for 10 MAPKKs while poplar and rice contain 13 MAPKKs each. The functions of these proteins have not been fully elucidated. There is evidence to suggest that MAPK cascades are involved in plant stress responses. In Arabidopsis, MKK3 plays a role in pathogen signaling; MKK2 is involved in cold and salt stress signaling; MKK4/MKK5 participates in innate immunity; and MKK7 regulates basal and systemic acquired resistance. The MAPKK subfamily is part of a larger superfamily that includes the catalytic domains of other STKs, protein tyrosine kinases, RIO kinases, aminoglycoside phosphotransferase, choline kinase, and phosphoinositide 3-kinase.