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J. Biol. Chem., Vol. 283, Issue 47, 32957-32967, November 21, 2008

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AtNOS/AtNOA1 Is a Functional Arabidopsis thaliana cGTPase and Not a Nitric-oxide Synthase^*

Magali Moreau, Gyu In Lee, Yongzeng Wang¹, Brian R. Crane, and Daniel F. Klessig²

From the Boyce Thompson Institute for Plant Research, Ithaca, New York 14853 and the Department of Chemistry and Chemical Biology, Cornell University, Ithaca, New York 14853

AtNOS1 was previously identified as a potential nitric-oxide synthase (NOS) in Arabidopsis thaliana, despite lack of sequence similarity to animal NOSs. Although the dwarf and yellowish leaf phenotype of Atnos1 knock-out mutant plants can be rescued by treatment with exogenous NO, doubts have recently been raised as to whether AtNOS1 is a true NOS. Moreover, depending on the type of physiological responses studied, Atnos1 is not always deficient in NO induction and/or detection, as previously reported. Here, we present experimental evidence showing that AtNOS1 is unable to bind and oxidize arginine to NO. These results support the argument that AtNOS1 is not a NOS. We also show that the renamed NO-associated protein 1 (AtNOA1) is a member of the circularly permuted GTPase family (cGTPase). AtNOA1 specifically binds GTP and hydrolyzes it. Complementation experiments of Atnoa1 mutant plants with different constructs of AtNOA1 show that GTP hydrolysis is necessary but not sufficient for the physiological function of AtNOA1. Mutant AtNOA1 lacking the C-terminal domain, although retaining GTPase activity, failed to complement Atnoa1, suggesting that this domain plays a crucial role in planta. cGTPases appear to be RNA-binding proteins, and the closest homolog of AtNOA1, the Bacillus subtilis YqeH, has been shown to participate in ribosome assembly and stability. We propose a similar function for AtNOA1 and discuss it in the light of its potential role in NO accumulation and plant development.

Received for publication, June 25, 2008 , and in revised form, September 8, 2008.

^* This work was supported, in whole or in part, by National Institutes of Health Grant 5R01GM067011 (to D. F. K.). This work was also supported by National Science Foundation Grant (to B. R. C.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

The on-line version of this article (available at http://www.jbc.org) contains supplemental Fig. S1.

¹ Present address: Genscript, 120 Centennial Ave., Piscataway, NJ 08854.

² To whom correspondence should be addressed: Boyce Thompson Institute for Plant Research, Ithaca, NY 14853. Tel.: 607-254-4560; Fax: 607-254-6779; E-mail: dfk8{at}cornell.edu.

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