In search of a ligand: What molecule activates receptor kinase SIRK1?

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Plants as sessile organisms are able to rapidly adapt to altering environmental conditions throughout the diurnal cycle. The integration of environmental and developmental signals in plants is often controlled by receptor kinases. The family of receptor kinases is the largest clade within the plant kinome consisting of over 600 members. These receptor kinases are activated by different types of ligands, such as carbohydrates, sterols or various types of small peptides. In the past years our group has characterized SIRK1 as a receptor kinase activated under alterations in external sucrose availability, but the molecule acting as a ligand to SIRK1 remains unknown. The major aim of this project is to identify candidate molecules that could act as a ligand to the receptor kinase SIRK1 and co-receptor SAK1. In the proposed experiments, we will use different approaches to identify potential ligand molecules. Thereby, we (i) will use biochemical methods fractionating low molecular weight protein components coupled to kinase activity assays, (ii) chimera of a receptor with known ligand and SIRK1 to confirm activity of the ligand candidates. Under the assumption that peptides could be under the ligand molecules, (iii) the study of subtilase knock-out mutants may give insights into the biogenesis of putative peptide ligands. In a complementary approach will (iv) search for putative ligands on a carbohydrate microarrays. Overall, the project will allow us to identify a new ligand for a receptor kinase involved in regulation of sucrose-induced responses.