Differential expression of two functional serine / threonine protein kinases from soyabean that have an unusual acidic domain at the carboxy terminus

Two soybean cDNA clones, SPK-3 and SPK- 4, encoding putative protein kinases were isolated and characterized. Both cDNAs encoded approximately 40-kDa serine/threonine kinases with unusual stretches of acidic amino acids in their carboxy-terminal regions, which are highly homologous to PKABA1 from wheat and ASKs from Arabidopsis. These kinases are encoded by one- or two-copy genes in the soybean genome. Notably, SPK-3 and -4 showed di€erent patterns of expression in various soybean tissues. SPK-3 is highly expressed in dividing and elongating tissues of young seedlings but relatively weakly in tissues of mature plants. In contrast, SPK-4 showed relatively high and constitutive expression in all the tissues examined except for leaf tissues of mature plants. Although various stressors, such as dehydration and high salinity, in- creased the expression of both genes, the induction ki- netics were di€erent. The two genes also di€ered in their response to abscisic acid (ABA). SPK-3 was induced but SPK-4 was not a€ected by exogenously supplied abscisic acid. In accordance with these expression data analysis of the activity of a chimeric SPK-3 promoter::b-glucu- ronidase (GUS) reporter gene by transient expression in tobacco leaves con®rmed the inducibility of SPK-3 by salt and ABA. Polyclonal antibodies raised against a recombinant SPK-4 protein produced in Escherichia coli speci®cally recognized both recombinant SPK-3 and -4 proteins. Kinase assays using anity-puri®ed SPK-4/ antibody complexes with crude soybean extracts as substrate identi®ed speci®c phosphorylation of two 41 and 170 kDa soybean proteins that were phosphorylat- ed on serine residues. Taken together, our results suggest that SPK-3, and/or SPK-4 are functional serine protein kinase(s). Furthermore, SPK-3 and -4 may play di€erent roles in the transduction of various environmental stresses.