Direct involvement of leucine-rich repeats in assembling ligand-triggered receptor-coreceptor complexes.

A common signaling mechanism of cell–cell and cell–environment communications in both animals and plants is mediated by receptor-like kinases (RLKs), which evolved independently in the two kingdoms but share a similar domain organization with a ligand-binding extracellular domain (ECD) connected via a single transmembrane helix to an intracellular kinase domain (KD) (1). The plant RLKs form a huge monophyletic protein superfamily with ∼440 and 790 members in Arabidopsis and rice, respectively (2). Based on sequence motifs in their ECDs, plant RLKs can be grouped into ∼20 families, with the largest family containing 1–32 tandem copies of leucine-rich repeat (LRR) (2), a widespread structural motif of ∼24 amino acids rich in leucine. These LRR-RLKs can be functionally classified into two major groups: the first group controls plant growth/development, such as BRI1, which perceives the plant steroid hormone brassinosteroids (BRs) (3), and the second group is involved in plant defense, including FLS2 and EFR, which recognize bacterial flagellin and translational elongation factor EF-Tu, respectively (4, 5). It is well known that ligand-induced homodimerization is a common mechanism to activate receptor kinases in animals (6). By contrast, plant LRR-RLKs, which likely exist as constitutive homodimers, are thought to be activated by ligand-induced heteromerization between a ligand-bound LRR-RLK and a non–ligand-binding LRRRLK (7). Little is known about how ligands trigger the formation of such receptor– coreceptor complexes, however. In PNAS, Jaillais et al. (8) at the Salk Institute report a significant discovery that provides a key advance in our understanding of the ligandinduced formation of receptor–coreceptor complexes. Among hundreds of plant LRR-RLKs, BAK1, having only five extracellular LRRs (eLRRs), is one of the most studied LRR-RLKs because of its multifunctionality in regulating both plant growth and defense (9). BAK1 was initially discovered as a coreceptor for the BR receptor BRI1 carrying 25 eLRRs (10, 11) and was later recognized as a coreceptor for the flagellin receptor FLS2 with 28 eLRRs (12, 13). Further studies revealed that BAK1 is also required for plant defense responses to other microbe-derived ligands [better known as microbe-associated molecular patterns (MAMPs)], including bacterial peptidoglycan and EF-Tu (12– 14). BAK1 does not directly participate in ligand binding or signal transduction but is rapidly recruited to ligand-bound BRI1, FLS2, and possibly other MAMPrecognition LRR-RLKs to activate their kinase/signaling activities fully via transphosphorylation (15, 16). The BRI1/FLS2BAK1 pairs have become paradigms for understanding the activation/signaling mechanisms of plant LRR-RLKs; however, little is known about what determines the binding specificity of the BRI1/ FLS2–BAK1 complexes. The study by Jaillais et al. (8) took advantage of a previously described gainof-function allele of BAK1 (bak1) (17) to reveal a crucial role of eLRRs in determining the binding specificity and driving the receptor-coreceptor interaction. The Arabidopsis elg (elongated) mutant was originally isolated as a suppressor of a dwarf mutant deficient in the plant growth hormone gibberellins (18) and was later found to carry an Asp(D)122-Asn(N) mutation in the third LRR of BAK1 responsible for a BR-hypersensitive phenotype (17). D122 is highly conserved between BAK1 and its homologs and is predicted to be a solvent-exposed residue on the concave surface of a curved solenoid LRR structure (19), which provides a surface for binding ligand/protein (20). Using a transgenic approach, Jaillais et al. (8) confirmed the stimulatory effect of bak1 on BR signaling and made a surprising discovery that the D122N mutation selectively affected several BAK1dependent immune responses, with bak1 blocking the plant immunity to peptidoglycan and flg22 (an active flagellinderived peptide) but having no effect on the EF-Tu–triggered plant defense. Such differential behaviors of bak1 on BR signaling/plant immunity were not caused by changes in the protein abundance or subcellular localization of BAK1, BRI1 or FLS2 but rather by altered affinity of bak1 to bind different LRR-RLKs. A coimmunoprecipitation experiment showed that bak1 failed to bind FLS2 in response to flg22 but interacted well with BRI1 even when the endogenous BR contents were below the level needed to maintain a detectable BRI1 binding to wild-type BAK1 (8). That a single amino acid change in an LRR motif prevented the recruitment of BAK1 to the flg22-bound FLS2 but enhanced the BRI1-BAK1 binding was a very significant discovery because it convincingly demonstrated a crucial role of eLRRs in selecting binding partners and in driving the formation of ligandinduced LRR–RLK complexes in plants. Previous studies suggested that BRinduced BRI1-BAK1 interaction was largely mediated by their KDs, which were known to interact in vitro and in yeast (10, 11, 15), whereas a recent model suggesting the ECD involvement in the FLS2BAK1 binding lacked any experimental support (19). The selective binding of BAK1 to different LRR-RLKs is consistent with a recent yeast two-hybrid study showing that BAK1 interacted with the LRR-containing ECD of a tomato LRR receptor-like protein, LeEix1, but failed to bind that of its closest homolog, LeEix2 (21). It is quite possible that ligand binding to an LRR-containing ECD alters its Fig. 1. A “double-lock” model for stabilizing a BR-triggered BRI1–BAK1 complex. Details are provided in the text. Double horizontal bars indicate the plasma membrane, the crescents mark the autoinhibitory C-terminal end, and the stars mark phosphorylation. (A) In the absence of BR, neither BRI1 nor BAK1 is active. The autoinhibitory C terminus of BRI1 and BKI1 binding to BRI1 prevents the BRI1–BAK1 interaction. (B) BR binding to the ligand-binding domain of BRI1 not only triggers a conformational change in its ECD to allow its lowaffinity dimerization with the BAK1 ECD but also causes a structural rearrangement in the BRI1 KD to activate its kinase activity. The slightly activated BRI1 autophosphorylates to lease its autoinhibitory C terminus and transphosphorylates to dissociate BKI1 from the plasma membrane, thus enabling physical docking of the KDs of BRI1 and BAK1 to form a stable receptor–coreceptor complex.