Abstract
Many flowering plants possess self-incompatibility (SI) system to prevent inbreeding. In Brassica self-incompatibility, self/non-self recognition is controlled by S-haplotype-specific interaction between the pollen-borne ligand, SP11 (S-locus protein 11), and its stigmatic receptor-kinase, SRK (S receptor kinase). Our genetic analysis of a self-compatible mutant revealed the involvement of a cytoplasmic protein kinase, MLPK (M-locus protein kinase) in the SI signaling. MLPK localizes to the papilla cell membrane and interacts directly with SRK on the plasma membrane to transduce SI signaling, but the regulation mechanism of MLPK kinase activity remains unclear.
To understand the activation mechanism of MLPK, we carried out an in vitro phosphorylation assay using E. coli-expressed recombinant MLPK and SRK protein. Our results indicated that N-terminal region of MLPK was not only the target specifically phosphorylated by SRK kinase domain but also the autoregulatory domain of MLPK itself.
