BIOCHEMICAL-PROPERTIES OF THE AUTOPHOSPHORYLATION OF RLK5, A RECEPTOR-LIKE PROTEIN-KINASE FROM ARABIDOPSIS-THALIANA

The RLK5 gene of Arabidopsis thaliana encodes a novel receptor-like pr otein kinase. DNA sequence analysis suggests that the RLK5 protein con tains an extracellular domain that has 21 tandemly repeated leucine-ri ch motifs linked, via a transmembrane hydrophobic region, to a protein kinase catalytic domain that is related to the serine/threonine famil y of protein kinases. To study the intrinsic biochemical properties of this protein kinase we have expressed the catalytic domain as two dif ferent recombinant fusion proteins in Escherichia coli. Both hybrid pr oteins have similar kinetic properties, autophosphorylate on serine an d threonine residues and have significantly greater activity in the pr esence of Mn2+ than Mg2+. A lysine to glutamic acid substitution in th e catalytic domain of RLK5 results in the catalytically inactive prote in RLK5(Cat)(K711E). The active RLK5 protein can phosphorylate the ina ctive K711E protein and the K711E protein can partially inhibit the au tophosphorylation of RLK5. Tryptic cleavage of the autophosphorylated proteins followed by two-dimensional thin layer electrophoresis indica tes that several sites in the catalytic domain are phosphorylated.