The activation path of PEP7: Understanding regulatory processes during maturation of PEP7 and in activation of the SIRK1/QSK1 receptor complex

• Applicants: Professor Dr. Klaus Harter; Professorin Dr. Waltraud Schulze
• Subject Area: Plant Biochemistry and Biophysics ; Plant Physiology ; Plant Cell and Developmental Biology
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 541328451

Plants as sessile organisms need to be able to rapidly adapt to altering abiotic and biotic environmental conditions throughout the diurnal cycle and during their life span. This requires precise integration of extracellular information with intracellular (metabolic) signals. The integration of environmental and developmental signals in plants is often controlled by small peptides which then activate signaling cascades through receptor kinases. Although peptide signaling and receptor kinases became a major research focus in the plant signaling field, detailed mechanisms on peptide maturation mechanisms and on the precise mechanisms of complex formation and activation at the plasma membrane are still not understood. Schulze lab has for many years been working on the receptor kinase SIRK1 which, when induced by sucrose, phosphorylates aquaporins to allow the plant cell to adjust to changing osmotic condition. Schulze lab has by means of protein biochemical methods identified QSK1 as a coreceptor modulating the activity of SIRK1. Harter lab has a long-lasting interest in dynamic complex formation of receptor kinases and has made important discoveries regarding pre-formed receptor-coreceptor complexes in membrane nanodomains. Recently, the Schulze and Harter lab have identified PEP7 as a ligand which activates the receptor-coreceptor complex SIRK1/QSK1. Using this clearly defined system of ligand (PEP7), receptor (SIRK1), coreceptor (QSK1) and substrate (aquaporin PIP2;4), this new project aims to unravel mechanisms during signal generation and receptor complex activation. Signal generation involves maturation of PEP7 from a propeptide, and preliminary work indicates potential regulatory mechanisms through propeptide phosphorylation. We therefore want to explore the role of these phosphorylation sites for PEP7 maturation and signaling, and we will search for kinases phosphorylating these sites. Receptor activation requires ligand recognition by receptor and coreceptor as well as close interaction of receptor/coreceptor and the substrate to initiate a phosphorylation cascade. Recent discovery of pre-formed receptor/coreceptor complexes questions the traditional activation model in which the ligand would actively induce complex formation. Using high-end methods of microscopy in combination with protein biochemistry we therefore want to further explore in vivo and biochemical SIRK1/QSK1 association in response to PEP7 supply and relate that to sucrose status of the plant.

• DFG Programme: Research Grants