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Published on April 22, 2009; 10.1104/pp.109.139170

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Received March 31, 2009
Accepted April 15, 2009

Auxin-responsive genes air12 code for a new family of plasma membrane b-type cytochromes specific to flowering plants

Valeria Preger *, Nunzio Tango , Christophe Marchand , Stephane D. Lemaire , Donatella Carbonera , Marilena Di Valentin , Alex Costa , Paolo Pupillo , and Paolo Trost

Laboratory of Molecular Plant Physiology, Department of Experimental Evolutionary Biology, University of Bologna, Via Irnerio 42, Bologna 40126, Italy; Institut de Biochimie et Biophysique Moleculaire et Cellulaire, UMR 8619; Institut de Biotechnologie des Plantes, UMR 8618, Batiment 630, CNRS, Univ Paris-Sud 11, Orsay F-91405 Cedex, France; Dipartimento di Scienze Chimiche, via Marzolo 1; Dipartimento di Biologia, via G. Colombo 3, Universita di Padova, Padova 35131, Italy

* Corresponding author; email: valeria.preger{at}unibo.it.

We here report on the identification of the major plasma membrane ascorbate-reducible b-type cytochrome of bean (Phaseolus vulgaris) and soybean (Glycine max) hypocotyls as orthologs of Arabidopsis AIR12 (Auxine-Induced in Root cultures). Soybean AIR12, which is glycosylated and glycosylphosphatidylinositol-anchored to the external side of the plasma membrane 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 {beta}-sandwich domain and belonging to the DOMON domain superfamily. It is shown to be a b-type cytochrome with a symmetrical {alpha}-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 EPR analysis indicate that AIR12 binds a single, highly axial low spin (HALS) heme, likely coordinated by Met-91 and His-76 which are strongly conserved in AIR12 sequences. Phylogenetic analyses reveal that the auxin-responsive genes AIR12 represent a new family of plasma membrane b-type cytochromes specific to flowering plants. Circumstantial evidence suggests that AIR12 may interact with other redox partners within the plasma membrane to constitute a redox link between cytoplasm and apoplast.






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