Phylogenetic analysis has previously shown that plant receptor-like kinases (RLKs) are monophyletic with respect to the kinase domain and share an evolutionary origin with the animal interleukin-1 receptor-associated kinase/Pellesoluble kinases. The lysin motif domain-containing receptorlike kinase-3 (LYK3) of the legume Medicago truncatula shows 33% amino acid sequence identity with human IRAK-4 over the kinase domain. Using the structure of this animal kinase as a template, homology modeling revealed that the plant RLK contains structural features particular to this group of kinases, including the tyrosine gatekeeper and the N-terminal extension ␣-helix B. Functional analysis revealed the importance of these conserved features for kinase activity and suggests that kinase activity is essential for the biological role of LYK3 in the establishment of the root nodule nitrogen-fixing symbiosis with rhizobia bacteria. The kinase domain of LYK3 has dual serine/threonine and tyrosine specificity, and mass spectrometry analysis identified seven serine, eight threonine, and one tyrosine residue as autophosphorylation sites in vitro. Three activation loop serine/ threonine residues are required for biological activity, and molecular dynamics simulations suggest that Thr-475 is the prototypical phosphorylated residue that interacts with the conserved arginine in the catalytic loop, whereas Ser-471 and Thr-472 may be secondary sites. A threonine in the juxtamembrane region and two threonines in the C-terminal lobe of the kinase domain are important for biological but not kinase activity. We present evidence that the structure-function similarities that we have identified between LYK3 and IRAK-4 may be more widely applicable to plant RLKs in general.Higher plants show a remarkable expansion of receptor-like kinases (RLKs) 4 ; for example, there are over 400 RLK genes in Arabidopsis thaliana (1), which have diverged to fulfill different physiological roles. These genes all encode proteins with the same basic structure as follows: an extracellular region (containing various domains that presumably bind different types of ligands) followed by a single transmembrane-spanning segment and an intracellular region containing a Ser/Thr kinaselike domain. Phylogenetic analysis has shown that the kinase domain is monophyletic and shares a common evolutionary origin with the human IRAK and Drosophila Pelle group of soluble Ser/Thr kinases. These kinases are quite closely related to the animal receptor Tyr kinase and the Raf kinase families (1).The structure of the human IRAK-4 kinase has been solved and has revealed several novel features, including a Tyr gatekeeper in the catalytic pocket, an N-terminal helix B, and regulation by activation loop phosphorylation that resembles Tyr kinases (2, 3). IRAK-4 contains an Arg before the catalytic Asp (an RD kinase), which interacts with a phosphorylated residue in the activation loop. It shares about 38% sequence identity with another family member, IRAK-1, which does not contain the Arg resi...

Background: Nod factor perception (NFP) protein is a plant, lysin motif receptor-like kinase. Results: Disulfide bridges that connect the three extracellular lysin motifs and the intracellular dead-kinase domain are essential for NFP function. Conclusion: Post-translational modifications are required for NFP folding, trafficking, and functioning. Significance: Structural information will help to determine NFP biochemical function.