Fig. 1.

A proposed action of a peptide hormone and its receptor–co-receptor pair in the ‘molecular glue’ model. (A) A receptor (red) interacts with its co-receptor (blue) only in the presence of a peptide hormone (brown). Upon binding of the peptide, it is considered that the receptor and co-receptor can phosphorylate each other, triggering the intracellular signaling. (B) The co-crystal structure of the IDA peptide (yellow) and LRR domains of its receptor HAESA (red) and co-receptor SERK1 (blue) (PBD accession number: 5IYX) (Santiago et al. 2016) is shown as an example of the peptide–receptor–co-receptor complex. The structure was illustrated using the NGL viewer in the Protein Data Bank website. (C) LRR proteins without kinase domains (green) may participate in direst recognition of ligands with LRR-RK receptors (red). Also, transmembrane kinases without extracellular domains (purple) may act to trigger intracellular signal transduction co-ordinately with LRR-RK receptors (red).