Structural basis for the impact of phosphorylation on the activation of plant receptor-like kinase BAK1

Receptor-like kinases (RLKs) constitute the major family of cell surface-associated receptors in plants and play essential roles in perceiving extracellular signals¹. Over two hundred members of the largest subfamily of RLKs that contain leucine-rich repeat extracellular domains (LRR-RLK) are found in Arabidopsis, among which BRI1-associated kinase 1 (BAK1) is one of the best studied. BAK1 was initially identified based on its association with the LRR-RLK BRI1, which perceives brassinosteroid, an important hormone that regulates a wide range of developmental and physiological processes in plants². BAK1 also serves as a co-receptor for several other LRR-RLKs that perceive pathogen-associated molecular patterns (PAMPs), including flagellin-sensing 2 (FLS2) and elongation factor EF-Tu receptor^3,4, and is therefore required for the innate immunity of plants. The reciprocal phosphorylation on the cytoplasmic domains of BAK1 and the ligand-binding RLKs in the complex is a prerequisite for the full activation of the receptor kinase⁵. The crystal structure of BAK1 complexed with AvrPtoB, which is an effector secreted by Pseudomonas syringae pv. Tomato to suppress PAMP-triggered immunity, recently revealed the mechanism by which BAK1 activity is inhibited⁶. However, the mechanism for BAK1 activation, particularly the impact of phosphorylation of key residues on BAK1 activation, remains unclear.

The core BAK1 kinase domain has the canonical bilobal structural architecture of a protein kinase family with N- and C-lobes. The extra N-terminal portion (G272-N291) forms a long extension, followed by a short helix (αB), which caps the top of the twisted β-sheets in the N-lobe. The C-lobe is predominantly α-helical, containing eight α-helices and two β-strands. The activation loop (D434-E462), which is restricted by the highly conserved DGF and APE motifs, attaches to the N-lobe and presents a fully extended architecture like that in other known active kinase structures^7,8. A well-ordered adenylyl imidodiphosphate (AMP-PNP) molecule binds with the canonical ATP-binding pocket. No cation is present in the structure, even though sufficient amounts of magnesium have been supplied in the purification and crystallization buffers.

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