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BIOCHEMICAL PROCESSES AND MACROMOLECULAR STRUCTURES

Auxin-Responsive Genes AIR12 Code for a New Family of Plasma Membrane b-Type Cytochromes Specific to Flowering Plants^{1,[C],[W],[OA]}

Laboratory of Molecular Plant Physiology, Department of Experimental Evolutionary Biology, University of Bologna, Bologna 40126, Italy (V.P., N.T., P.P., P.T.); Institut de Biochimie et Biophysique Moléculaire et Cellulaire, UMR 8619 (C.M.), and Institut de Biotechnologie des Plantes, UMR 8618 (S.D.L.), CNRS, Université Paris-Sud 11, Orsay F–91405 cedex, France; and Dipartimento di Scienze Chimiche (D.C., M.D.V.) and Dipartimento di Biologia (A.C.), Università di Padova, Padova 35131, Italy

We report here on the identification of the major plasma membrane (PM) ascorbate-reducible b-type cytochrome of bean (Phaseolus vulgaris) and soybean (Glycine max) hypocotyls as orthologs of Arabidopsis (Arabidopsis thaliana) AIR12 (for auxin induced in root cultures). Soybean AIR12, which is glycosylated and glycosylphosphatidylinositol-anchored to the external side of the PM in vivo, was expressed in Pichia pastoris in a recombinant form, lacking the glycosylphosphatidylinositol modification signal and purified from the culture medium. Recombinant AIR12 is a soluble protein predicted to fold into a β-sandwich domain and belonging to the DOMON (for dopamine β-monooxygenase N terminus) domain superfamily. It is shown to be a b-type cytochrome with a symmetrical -band at 561 nm, fully reduced by ascorbate, and fully oxidized by monodehydroascorbate radical. AIR12 is a high-potential cytochrome b showing a wide bimodal dependence from the redox potential between +80 mV and +300 mV. Optical absorption and electron paramagnetic resonance analysis indicate that AIR12 binds a single, highly axial low-spin heme, likely coordinated by methionine-91 and histidine-76, which are strongly conserved in AIR12 sequences. Phylogenetic analyses reveal that the auxin-responsive genes AIR12 represent a new family of PM b-type cytochromes specific to flowering plants. Circumstantial evidence suggests that AIR12 may interact with other redox partners within the PM to constitute a redox link between cytoplasm and apoplast.

² These authors contributed equally to the article.

The author responsible for the 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: Valeria Preger (valeria.preger{at}unibo.it).

^[C] Some figures in this article are displayed in color online but in black and white in the print edition.

^[W] The online version of this article contains Web-only data.

^[OA] Open access articles can be viewed online without a subscription.

www.plantphysiol.org/cgi/doi/10.1104/pp.109.139170

^* Corresponding author; e-mail valeria.preger{at}unibo.it.

Received March 31, 2009; accepted April 15, 2009; published April 22, 2009.