Submitted on September 11, 2009
Accepted on October 9, 2009
Structural Mechanism of Abscisic Acid Binding and Signaling by Dimeric PYR1
Noriyuki Nishimura 1
, Kenichi Hitomi 2, Andrew S. Arvai 3, Robert P. Rambo 4, Chiharu Hitomi 3, Sean R. Cutler 5, Julian I. Schroeder 1, Elizabeth D. Getzoff 3*
1 Division of Biological Sciences, Cell and Developmental Biology Section, University of California at San Diego, La Jolla, CA 92093, USA.
2 Department of Molecular Biology and The Skaggs Institute for Chemical Biology, The Scripps Research Institute, La Jolla, CA 92037, USA.; Life Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA.
3 Department of Molecular Biology and The Skaggs Institute for Chemical Biology, The Scripps Research Institute, La Jolla, CA 92037, USA.
4 Life Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA.
5 Department of Botany and Plant Sciences, Center for Plant Cell Biology, University of California at Riverside, Riverside, CA 92521, USA.
* To whom correspondence should be addressed.
Elizabeth D. Getzoff , E-mail: edg{at}scripps.edu
These authors contributed equally to this work.
The phytohormone abscisic acid (ABA) acts in seed dormancy, plant development, drought tolerance, and adaptive responses to environmental stresses. Structural mechanisms mediating ABA receptor recognition and signaling remain unknown, but are essential for understanding and manipulating abiotic stress resistance. Here, we report structures of PYR1, a prototypical PYR/PYL/RCAR protein that functions in early ABA signaling. The crystallographic structure reveals an 
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helix-grip fold and homodimeric assembly, verified in vivo by co-immunoprecipitation. ABA binding within a large internal cavity switches structural motifs distinguishing ABA-free "open-lid" from ABA-bound "closed-lid" conformations. Small angle x-ray scattering suggests that ABA signals by converting PYR1 to a more compact, symmetric closed-lid dimer. Site-directed PYR1 mutants designed to disrupt hormone binding lose ABA-triggered interactions with type 2C protein phosphatase partners in planta.
