Protéines membranaires dans les interactions pollen-pistil

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The integral membrane S-locus receptor kinase of Brassica has serine/threonine kinase activity in a membranous environment and spontaneously forms oligomers in planta

To gain further insight into the mode of action of S-locus receptor kinase (SRK), a receptor-like kinase involved in the self-incompatibility response in Brassica, different recombinant SRK proteins have been expressed in a membranous environment using the insect cell/baculovirus system. Recombinant SRK proteins exhibited properties close to those of the endogenous stigmatic SRK protein and were found to autophosphorylate on serine and threonine residues in insect cell microsomes. Autophosphorylation was constitutive because it did not require the presence of pollen or stigma extracts in the phosphorylation buffer. Phosphorylation was shown to occur in trans, suggesting the existence of constitutive homooligomers of membrane-anchored recombinant SRK. To investigate the physiological relevance of these results, we have examined the oligomeric status of SRK in planta in cross-linking experiments and by velocity sedimentation on sucrose gradients. Our data strongly suggest that SRK is associated both with other SRK molecules and other stigma proteins in nonpollinated flowers. These findings may have important implications for our understanding of self-pollen signaling

Membrane proteins involved in pollen-pistil interactions

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The S locus receptor kinase and the self-incompatibility response in Brassica

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The S-locus receptor kinase is inhibited by thioredoxins and activated by pollen coat proteins

International audienceThe self-incompatibility response in Brassica allows recognition and rejection of self-pollen by the stigmatic papillae. The transmembrane S-locus receptor kinase (SRK), a member of the receptor-like kinase superfamily in plants, mediates recognition of self-pollen on the female side1, whereas the S-locus cysteine-rich protein (SCR) is the male component of the self-incompatibility response2. SCR is presumably located in the pollen coat, and is thought to be the SRK ligand2,3. Although many receptor-like kinases have been isolated in plants, the mechanisms of signal transduction mediated by these molecules remain largely unknown. Here we show that SRK is phosphorylated in vivo within one hour of self-pollination. We also show that, in vitro, autophosphorylation of SRK is prevented by the stigma thioredoxin THL1 in the absence of a ligand. This inhibition is released in a haplotype-specific manner by the addition of pollen coat proteins. Our data indicate that SRK is inhibited by thioredoxins and activated by pollen coat proteins

Characterization of the S locus genes, SLG and SRK, of the Brassica S3 haplotype: identification of a membrane-localized protein encoded by the S locus receptor kinase gene

International audienceThe S locus, which controls the self-incompatibility response in Brassica, has been shown to contain at least two genes. SLG encodes a secreted S locus glycoprotein whilst SRK encodes a putative S locus receptor kinase. SRK has been shown potentially to encode a functional kinase and genetic evidence indicates that this gene is essential for the self-incompatibility response. Here the characterization of the SRK and SLG genes of a Brassice line homozygous for the S3 haplotype is described. A 120 kDa glycoprotein was identified in stigmas and several lines of evidence indicated that this protein is encoded by the SRK 3 gene. First, the 120 kDa glycoprotein was recognized by antibodies raised against peptides based on the SRK 3 gene sequence. Secondly, this protein is polymorphic and, in an F 2 population segregating for the S3 haplotype, was expressed only in plants possessing the S3 haplotype. Thirdly, the 120 kDa protein was expressed specifically in stigmas. Finally, the 120 kDa protein was only extracted from stigmas in the presence of detergent indicating that it is anchored in the membrane. SRK has been predicted to encode a transmembrane glycoprotein based on the deduced amino acid sequence. Located on the membrane, SRK is in a position to interface between an extracellular recognition event between pollen and pistil and an intracellular signal transduction pathway which initiates the self-incompatibility response