Abstract
Many flowering plants possess a self-incompatibility system to prevent inbreeding. In Brassica rapa, self/non-self recognition in mating is established through S-haplotype-specific interactions between stigma receptors and S-locus protein 11 (SP11, also called S-locus cysteine-rich protein) that is encoded at the highly polymorphic S-locus. Here we describe the solution structure of the SP11 protein of the S8-haplotype (S8-SP11), which specifically binds to the stigma factor of the same haplotype. It folds into an alpha/beta 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 SP11 identified a hyper-variable (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.
Highlights
Many flowering plants possess a self-incompatibility system to prevent inbreeding
Structure-based sequence alignment and homology modeling of allelic SP11 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
Based on the structure and structure-based sequence alignment of allelic SP11s of different S-haplotypes, we identified a loop region that might determine the allele-specific binding to the stigma receptors
Summary
Many flowering plants possess a self-incompatibility system to prevent inbreeding. In Brassica rapa, self/ non-self recognition in mating is established through S-haplotype-specific interactions between stigma receptors and S-locus protein 11 (SP11, called S-locus cysteine-rich protein) that is encoded at the highly polymorphic S-locus. Among these pollen and stigma factors, SRK and SLG, the female factors, exhibited several features typical of SI factors They are predominantly expressed in stigma papilla cells immediately prior to flower opening concomitant with the acquisition of SI in the stigma. Following the accumulation of such circumstantial evidence, “gain-of-function” experiments employing transgenics established that SRK alone determines the S-haplotype specificity of the stigma, whereas SLG facilitates the recognition process [7]. In contrast to these female factors, the identification of the male S determinant remained elusive. Confirmation was provided through the use of transgenic gainof-function experiments and a pollination bioassay using purified SP11 [3, 9]
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