Fig. 8: Adaptation of LRR^{ID + 4LRR} to differential downstream signalling pathways.

a Design of AtRLP23-BRI1 chimeras. Different regions (cytosolic, TM+cytosolic, and eJM+TM+cytosolic) of BRI1 were swapped into AtRLP23 as indicated. The alignment of amino acids in the eJM and TM regions from AtRLP23 and AtBRI1 is shown. Amino acid residues with negative charges are in red and amino acids with positive charges are in blue. The GxxxG motif in the TM region is highlighted in yellow. b, e Immuno-precipitation to test interactions between AtRLP23 chimeras, AtBAK1 and AtSOBIR1. Nb leaves expressing the indicated constructs were treated with either mock or 1 μM nlp20 for 5 min. c, d, f, g Functionality testing of AtRLP23 chimeras. Nb leaves expressing the indicated constructs were treated with 1 μM nlp20 and samples were collected at indicated time points. Dephosphorylation of BESI1-HA was detected with HA antibody. Phosphorylation of NbSIPK and NbWIPK was detected with p-P42/44 antibody. For f, twice the sample of RLP23^oe-BRI1 was loaded as a reference, because RLP23^oe-BRI1 protein accumulation is weaker than that of RLP23^WT. d, g Nb leaf discs expressing the indicated constructs (without BES1-HA) were collected and treated with either mock or 1 μM nlp20 and ROS production was measured for 90 min. For d and g, solid line, mean; shaded band, s.e.m. RLU, relative light units. For details of experiential design, refer to the methods section. h Schematic model of the interaction between LRR-RLK-Xb^ID+4LRR and LRR-RLP^ID + ^4LRR with co-receptors to induce differential downstream signalling. Both receptor classes utilise the last 4 LRRs (highlighted in yellow) to interact with SERKs (BAK1). LRR-RLP evolved to interact with SOBIR1 with the GxxxG motifs in TM (highlighted in yellow outline). Coloured hexagons on RLKs indicate activated kinases and black hexagon indicates an inactivated kinase. i Modular evolution of different domains in cell-surface receptors to allow diverse ligand recognition and specificity of downstream signalling. Domains or regions that evolved different functions are highlighted in yellow. Bold arrows represent large expansions and diversifications. K* represents the lysine in Kx₅Y or Yx₈KG motifs in ID from LRR-RLPs. Domain or region structures (from left to right) are obtained from: BRI1 ectodomain (3RGX); RXEG1 ectodomain (7W3X); PSKR1 ID (4Z63); RXEG1 ID (7W3X); PSKR1 C3 (4Z63); RXEG1 C3 (7W3X); PSKR1 ejM-TM (predicted from Alphafold2²⁰); RLP23 eJM-TM (predicted from Alphafold2²⁰); BRI1 kinase (4OH4). Structures were visualized in iCn3D⁹⁸. For b, c, e and f, the experiments were repeated at least twice with similar results.