SRK is a plant receptor kinase involved in the self-incompatibility system of Brassica species. During a cDNA screening for the phosphoproteins from a stigma expression library, a clone encoding the nucleoside diphosphate kinase III (Bc-NDPK III) was obtained. After in vitro phosphorylation assays with recombinant proteins, Bc-NDPK III contained mostly phosphoserine. By contrast, the kinase domain of SRK contained phosphoserine and phosphothreonine, both of which were significantly increased by the addition of Bc-NDPK III in the presence of an SRK inhibitor KN-62. The result suggested the possible involvement of Bc-NDPK III in the signal transduction pathway through SRK.
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