Smoke-derived karrikin perception by the α/β-hydrolase KAI2 from Arabidopsis

Genetic studies in Arabidopsis implicate an alpha/beta-hydrolase, KARRIKIN-INSENSITIVE 2 (KAI2) as a receptor for karrikins, germination-promoting butenolide small molecules found in the smoke of burned plants. However, direct biochemical evidence for the interaction between KAI2 and karrikin and for the mechanism of downstream signaling by a KAI2-karrikin complex remain elusive. We report crystallographic analyses and ligand-binding experiments for KAI2 recognition of karrikins. The karrikin-1 (KAR(1)) ligand sits in the opening to the active site abutting a helical domain insert but distal from the canonical catalytic triad (Ser95-His246-Asp217) of alpha/beta-hydrolases, consistent with the lack of detectable hydrolytic activity by purified KAI2. The closest approach of KAR(1) to Ser95-His246-Asp217 is 3.8 angstrom from His246. Six aromatic side chains, including His246, encapsulate KAR(1) through geometrically defined aromatic-aromatic interactions. KAR(1) binding induces a conformational change in KAI2 at the active site entrance. A crevice of hydrophobic residues linking the polar edge of KAR(1) and the helical domain insert suggests that KAI2-KAR(1) creates a contiguous interface for binding signaling partners in a ligand-dependent manner.