Crystal structure of a plant leucine rich repeat protein with two island domains.

Leucine rich repeat (LRR) domain, characterized by a repetitive sequence pattern rich in leucine residues, is a universal protein-protein interaction motif present in all life forms. LRR repeats interrupted by sequences of 30-70 residues (termed island domain, ID) have been found in some plant LRR receptor-like kinases (RLKs) and animal Toll-like receptors (TLR7-9). Recent studies provide insight into how a single ID is structurally integrated into an LRR protein. However, structural information on an LRR protein with two IDs is lacking. The receptor-like protein kinase 2 (RPK2) is an LRR-RLK and has important roles in controlling plant growth and development by perception and transduction of hormone signal. Here we present the crystal structure of the extracellular LRR domain of RPK2 (RPK2-LRR) containing two IDs from Arabidopsis. The structure reveals that both of the IDs are helical and located at the central region of the single RPK2-LRR solenoid. One of them binds to the inner surface of the solenoid, whereas the other one mainly interacts with the lateral side. Unexpectedly, a long loop immediately following the N-terminal capping domain of RPK2-LRR is presented toward and sandwiched between the two IDs, further stabilizing their embedding to the LRR solenoid. A potential ligand binding site formed by the two IDs and the solenoid is located at the C-terminal side of RPK2-LRR. The structural information of RPK2-LRR broadens our understanding toward the large family of LRR proteins and provides insight into RPK2-mediated signaling.