2 MEMBERS OF THE THIOREDOXIN-H FAMILY INTERACT WITH THE KINASE DOMAINOF A BRASSICA-S LOCUS RECEPTOR KINASE

To determine potential targets of the S locus receptor kinase (SRK) du ring the Brassica self-incompatibility response, a yeast two-hybrid li brary was screened with the SRK-910 protein kinase domain. Two thiored oxin-h-like clones, THL-1 and THL-2, were found to interact specifical ly with the SRK-910 protein kinase domain and not to interact with the protein kinase domains from the Arabidopsis receptor-like protein kin ases (RLK) RLK4 and RLK5, The interaction between THL-1 and the SRK-91 0 protein kinase domain was confirmed using coimmunoprecipitation expe riments with fusion proteins produced in Escherichia coli. THL-1 has t hioredoxin activity based on an insulin reduction assay, and THL-1 is weakly phosphorylated by the SRK-910 protein kinase domain. THL-1 and THL-2 are both expressed in a variety of tissues but show some differe nces in steady state mRNA levels, with THL-2 being preferentially expr essed in floral tissues. This indicates a more general biological func tion for these thioredoxins in addition to a potential role as effecto r molecules in the self-incompatibility signal cascade.