A receptor-like protein kinase with a lectin-like domain from lombardy poplar: gene expression in response to wounding and characterization of phosphorylation activity

Plant receptor-like protein kinases (RLKs) are thought to be involved in various cellular processes mediated via signal transduction pathways. To clarify the initial step in such a signal transduction pathway in woody plants, we cloned a cDNA encoding PnLPk (a Populus nigra var. italica lectin-like protein kinase) from lombardy poplar. The C-terminal region of the predicted PnLPK protein includes a protein kinase catalytic domain consisting of the 12 subdomains typical of the eukaryotic protein kinase superfamily. Following the signal peptide at the N-terminus, a domain that shows homology to legume lectins retains the putative Mn2+ and Ca2+-binding amino acids, which are highly conserved among lectin-related proteins. Because a putative hydrophobic transmembrane domain was localized between the lectin-like domain and the protein kinase domain, PnLPK was determined to be a member of the plant RLK subfamily with a lectin-like domain. Transcripts of the PnLPk gene accumulate in roots. mature leaves and caili of lombardy poplar, whereas only trace amounts of the transcripts are detectable in stems, young leaves and apical buds. Wounding of the young leaves increased the amount of PnLPK mRNA, but none of several phytohormones tested had any effect on the transcription of PnLPK. When incubated in the presence of divalent metal cations such as Mn2+, the C-terminal catalytic domain of PnLPK showed significantly higher autophosphorylation activity than the full-length PnLPK protein. The phosphorylation activity of PnLPK was also detected using beta-casein as substrate. Phosphoamino acid analysis indicated that PnLPK is a serine/threonine kinase.