S-RNase proteins: functional studies of the 120kDa glycoprotein and SRNase oligomerization
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Flowering plants control fertilization through pollen-pistil interactions. Self-incompatibility (SI) is a well-studied pollen-pistil interaction that promotes cross-pollination. SI is controlled by a multi-haplotype locus called the S-locus. In Nicotiana alata, S-RNase is a product of the S-locus and regulates specificity in the pistil, while S-locus F-box protein (SLF) controls specificity in the pollen. The interaction between S-RNase and SLF determines whether the pollination is compatible or incompatible. In an incompatible cross, the ribonuclease activity of S-RNase inhibits pollen tube growth. Genetic experiments indicate that, in addition to S-RNase and SLF, non-S-factors are also required for SI. S-RNase binding proteins represent potential non-S-factors required for SI. Using affinity chromatography, we found that S-RNase selfassociates and three homologous stylar glycoproteins - the 120kDa glycoprotein (120K), N. alata pistil extensin-like protein III (NaPELP III), and N. alata transmitting tract specific glycoprotein (NaTTS) - bind directly to S-RNase. I studied the oligomerization of S-RNase in detail and found that self-association is dependent on S-haplotype and buffer conditions. I determined that the components of the S-RNase complex account for 30% of soluble pistil protein. 120K is the most likely candidate for a non-S-factor because it enters the cytoplasm of growing pollen tubes and shows polymorphism when SI and self-compatible Nicotiana species are compared. To test its role in SI, I suppressed 120K expression using RNAi. Suppressing 120K caused a breakdown of SI, confirming that it functions in SI.