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First published online February 17, 2006; 10.1104/pp.105.073510 Plant Physiology 140:1233-1245 (2006) © 2006 American Society of Plant Biologists OPEN ACCESS ARTICLE
Mutations in the NB-ARC Domain of I-2 That Impair ATP Hydrolysis Cause Autoactivation1,[OA]Plant Pathology (W.I.L.T., J.H.V., B.J.C.C., F.L.W.T.), Biomolecular Mass Spectrometry (J.A.B.), and Plant Physiology (M.A.H.), Swammerdam Institute for Life Sciences, University of Amsterdam, 1090 GB Amsterdam, The Netherlands; and Max Planck Institute for Informatics, 66123 Saarbruecken, Germany (M.A., T.L.)
Resistance (R) proteins in plants confer specificity to the innate immune system. Most R proteins have a centrally located NB-ARC (nucleotide-binding adaptor shared by APAF-1, R proteins, and CED-4) domain. For two tomato (Lycopersicon esculentum) R proteins, I-2 and Mi-1, we have previously shown that this domain acts as an ATPase module that can hydrolyze ATP in vitro. To investigate the role of nucleotide binding and hydrolysis for the function of I-2 in planta, specific mutations were introduced in conserved motifs of the NB-ARC domain. Two mutations resulted in autoactivating proteins that induce a pathogen-independent hypersensitive response upon expression in planta. These mutant forms of I-2 were found to be impaired in ATP hydrolysis, but not in ATP binding, suggesting that the ATP- rather than the ADP-bound state of I-2 is the active form that triggers defense signaling. In addition, upon ADP binding, the protein displayed an increased affinity for ADP suggestive of a change of conformation. Based on these data, we propose that the NB-ARC domain of I-2, and likely of related R proteins, functions as a molecular switch whose state (on/off) depends on the nucleotide bound (ATP/ADP).
1 This work was supported by Bioseeds/Keygene (J.V.), the German Research Foundation (contract no. LE 491/14–1 to M.A. and T.L.), the German National Genome Research Network, and the BioSapiens Network of Excellence funded by the European Commission (grant no. LSHG–CT–2003–503265). 2 These authors contributed equally to the paper. 3 Present address: The Sainsbury Laboratory, John Innes Centre, Norwich Research Park, Colney, Norwich NR4 7UH, UK. 4 Present address: Laboratory of Phytopathology, Wageningen University, Binnenhaven 5, 6709 PD Wageningen, The Netherlands. The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Frank L.W. Takken (takken{at}science.uva.nl). [OA] Open Access articles can be viewed online without a subscription. Article, publication date, and citation information can be found at www.plantphysiol.org/cgi/doi/10.1104/pp.105.073510. * Corresponding author; e-mail takken{at}science.uva.nl; fax 31–20–5257934. Received October 28, 2005; returned for revision January 25, 2006; accepted January 26, 2006. This article has been cited by other articles:
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