%D 2002 %0 ARTICLE %T Biochemical characterization of the kinase domain of the rice disease resistance receptor-like kinase XA21 . %J J Biol . Chem . %V 277 ( 23 ) %P 20264-9 %A Liu GZ %A Pi LY %A Walker JC %A Ronald PC %A Song WY %M pub11927577 %X The rice disease resistance gene , Xa21 , encodes a receptor kinase-like protein consisting of leucine-rich repeats in the putative extracellular domain and a serine/threonine kinase in the putative intracellular domain . The putative XA21 kinase domain was expressed as maltose-binding and glutathione S-transferase fusion proteins in Escherichia coli . The fusion proteins are capable of autophosphorylation . Phosphoamino acid analysis of the glutathione S-transferase fusion protein indicates that only serine and threonine residues are phosphorylated . The relative phosphorylation rate of the XA21 kinase against increasing enzyme concentrations follows a first-order rather than second-order kinetics , indicating an intramolecular phosphorylation mechanism . Moreover , the active XA21 kinase can not phosphorylate a kinase-deficient mutant of XA21 kinase . The enzymatic activity of the XA21 kinase in a buffer containing Mn ( 2+ ) is at least 15 times higher than that with Mg ( 2+ ) . The K ( m ) and V ( max ) of XA21 kinase for ATP are 0 . 3 microm and 8 . 4 nmol/mg/min , respectively . Tryptic phosphopeptide mapping reveals that multiple sites on the XA21 kinase are phosphorylated . Finally , our data suggest that the region of XA21 kinase corresponding to the RD kinase activation domain is not phosphorylated , revealing a distinct mode of action compared with the tomato Pto serine/threonine kinase conferring disease resistance .