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BIOCHEMICAL PROCESSES AND MACROMOLECULAR STRUCTURES

Molecular Identification of an Arabidopsis S-Adenosylmethionine Transporter. Analysis of Organ Distribution, Bacterial Expression, Reconstitution into Liposomes, and Functional Characterization¹

Department of Pharmaco-Biology, Laboratory of Biochemistry and Molecular Biology, University of Bari, 70125 Bari, Italy (L.P., R.A., E.B., F.P.); Consiglio Nazionale delle Ricerche, Institute of Biomembranes and Bioenergetics, 70125 Bari, Italy (L.P., R.A., F.P.); and Department Willmitzer, Max-Planck-Institut für Molekulare Pflanzenphysiologie, Potsdam-Golm 14476, Germany (F.C., M.I.Z., C.S.-G., A.R.F.)

Despite much study of the role of S-adenosylmethionine (SAM) in the methylation of DNA, RNA, and proteins, and as a cofactor for a wide range of biosynthetic processes, little is known concerning the intracellular transport of this essential metabolite. Screening of the Arabidopsis (Arabidopsis thaliana) genome yielded two potential homologs of yeast (Saccharomyces cerevisiae) and human SAM transporters, designated as SAMC1 and SAMC2, both of which belong to the mitochondrial carrier protein family. The SAMC1 gene is broadly expressed at the organ level, although only in specialized tissues of roots with high rates of cell division, and appears to be up-regulated in response to wounding stress, whereas the SAMC2 gene is very poorly expressed in all organs/tissues analyzed. Direct transport assays with the recombinant and reconstituted SAMC1 were utilized to demonstrate that this protein displays a very narrow substrate specificity confined to SAM and its closest analogs. Further experiments revealed that SAMC1 was able to function in uniport and exchange reactions and characterized the transporter as highly active, but sensitive to physiologically relevant concentrations of S-adenosylhomocysteine, S-adenosylcysteine, and adenosylornithine. Green fluorescent protein-based cell biological analysis demonstrated targeting of SAMC1 to mitochondria. Previous proteomic analyses identified this protein also in the chloroplast inner envelope. In keeping with these results, bioinformatics predicted dual localization for SAMC1. These findings suggest that the provision of cytosolically synthesized SAM to mitochondria and possibly also to plastids is mediated by SAMC1 according to the relative demands for this metabolite in the organelles.

² These authors contributed equally to the paper.

³ Present address: Instituto de Biotecnologia, Centro de Investigación en Ciencias Veterinarias y Agronómicas, Instituto Nacional de Tecnologia Agricola B1712WAA, Castelar, Buenos Aires, Argentina.

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: Ferdinando Palmieri (fpalm{at}farmbiol.uniba.it).

www.plantphysiol.org/cgi/doi/10.1104/pp.106.086975

^* Corresponding author; e-mail fpalm{at}farmbiol.uniba.it; fax 39–080–544–2770.

Received July 19, 2006; accepted August 29, 2006; published September 1, 2006.

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