| Research Abstract |
1.Solution structure of the pollen ligand, SP11
Many flowering plants possess a self-incompatibility (SI) system to prevent inbreeding. In Brassica, self/non-self recognition in mating is established through S-haplotype specific interactions between stigmatic receptor kinase, SPK, and its pollen ligand, SP11. In this study, we determined the solution structure of the SP11 protein of the S_8-haplotype (S_8-SP11) by using NMR spectroscopy. It folds into an α/β sandwich structure that resembles those of plant defensins. Residues important for structural integrity are highly conserved among the allelic SP11s, suggesting the existence of a common folding pattern. Structure-based sequence alignment and homology modeling of allelic SP11s identified a hypervariable (HV) region, which is thought to form a loop that bulges out from the body of the protein that is amenable to solvent exposure. We suggest that the HV region could serve as a specific binding site for the stigma receptor.
2.Finding of MLPK, a possible component of the stigmatic receptor complex.
SI response in Brassica is initiated by haplotype-specific interactions between the pollen-bome ligand SP11 and its stigmatic receptor kinase SRK. This binding induces autophosphorylation of SRK, which is then thought to trigger a signaling cascade that leads to self-pollen rejection. A recessive mutation of the modifier (m) gene eliminates the SI response in stigma. Positional cloning of M has revealed that it encodes a membrane-anchored cytoplasmic serine/threonine protein kinase, designated MLPK. Transient expression of MLPK restores the ability of mm papilla cells to reject self-pollen, suggesting that MLPK is a positive mediator of Brassica SI signaling. The plasma membrane localization of MLPK suggests that it may function in the vicinity of SRK. MLPK, acting within the SRK receptor complex, is an attractive model for the future study of SI signaling.
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