PSY1R belongs to the family of plant leucine-rich repeat receptor-like kinases that play important roles in processes such as growth regulation and plant immunity response. PSY1R was proposed to be the receptor of the plant peptide hormone PSY1 which promotes cell expansion. PSY1R was furthermore shown to phosphorylate and regulate the activity of the plasma membrane localized H+-ATPase, AHA2. While the mechanism of PSY1R-mediated AHA2 phosphorylation has previously been studied in detail, little is known about how PSY1R binds PSY1 peptide ligand and how the intracellular PSY1R kinase domain is activated.
This work provides the first study of the direct interaction between PSY1R and the peptide ligand PSY1. The binding was evaluated both for full length PSY1R expressed in plants and for the isolated extracellular domain expressed in insect cells. PSY1 binds to the extracellular domain of PSY1R with a Kd of approximately 17 nM which is physiologically relevant.
The activation mechanism of PSY1R was also studied. Full length PSY1R forms homodimers and interacts with members of the SERK co-receptor family in planta. An intact PSY1R C-terminal domain was shown to be required for these interactions. Furthermore, deletion of the C-terminal domain of PSY1R abolishes kinase activity. A kinase activation mechanism that relies on C-terminal domain-mediated dimerization represents a novel kinase regulatory mechanism. PSY1R autophosphorylates on serine, threonine and tyrosine residues and mass spectrometry-based autophosphorylation site mapping led to the discovery of seven in vitro autophosphorylation sites within the kinase domain of PSY1R. PSY1R was also shown to autophosphorylate in trans. Initial transphosphorylation takes place within the kinase activation loop at residues Ser951, Thr959 and Thr963. Thr959 and Thr963 are conserved among related plant leucine-rich repeat receptor-like kinases whereas Ser951 is unique for PSY1R which suggests that it may serve a specialized function in regulation of PSY1R kinase activity.