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Smoke-derived karrikin perception by the α/β-hydrolase KAI2 from Arabidopsis
Wednesday, 2013/05/15 | 08:33:05

Yongxia Guoa,b,1, Zuyu Zhenga,c,1, James J. La Claird,1, Joanne Chorya,c,2, and Joseph P. Noela,b,2

Author Affiliations

1.aHoward Hughes Medical Institute,
2.bJack H. Skirball Center for Chemical Biology and Proteomics, and
3.cPlant Biology Laboratory, Salk Institute for Biological Studies, La Jolla, CA 92037; and
4.dDepartment of Chemistry and Biochemistry, University of California, San Diego, CA 92093

1.Contributed by Joanne Chory, April 2, 2013 (sent for review March 18, 2013)




Genetic studies in Arabidopsis implicate an α/β-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 (KAR1) 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 α/β-hydrolases, consistent with the lack of detectable hydrolytic activity by purified KAI2. The closest approach of KAR1 to Ser95-His246-Asp217 is 3.8 Å from His246. Six aromatic side chains, including His246, encapsulate KAR1 through geometrically defined aromatic–aromatic interactions. KAR1 binding induces a conformational change in KAI2 at the active site entrance. A crevice of hydrophobic residues linking the polar edge of KAR1 and the helical domain insert suggests that KAI2–KAR1 creates a contiguous interface for binding signaling partners in a ligand-dependent manner.



PNAS May 14, 2013 vol. 110 no. 20 8284-8289

Fig. 4. Conformational changes in KAI2 induced by KAR1:  (B) Close-up ribbon diagrams of the KAR1-binding site depicting KAI2 in the absence (teal) and presence (purple) of KAR1superimposed. Phe134 and Phe194 are shown as teal (apo) or purple (complex) sticks. KAR1 carbons are yellow and oxygen is red



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