Molecular identification of an Arabidopsis thaliana S-adenosylmethionine transporter: analysis of organ distribution, bacterial expression, reconstitution into liposomes and functional characterization

PALMIERI, L.; ARRIGONI, R.; BLANCO, E.; CARRARI, FERNANDO OSCAR; RODRIGUEZ STUDART, CLAUDIA; ALISDAIR FERNIE,; PALMIERI, F.

Plant Physiology

Año: 2006 vol. 142 p. 855 - 865

0032-0889

Despite much study of the role of S-adenosylmethionine in the methylation of DNA, RNA and proteins and as a co-factor for a wide range of biosynthetic processes, little is known concerning the intracellular transport of this essential metabolite. Screening of the Arabidopsis thaliana genome yielded two potential homologs of yeast and human S-adenosylmethionine 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 specialised tissues of roots with high rates of cell division, and appears to be upregulated 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 S-adenosylmethionine and its closest analogs. Further experiments revealed that SAMC1 was able to function in uniport and exchange reactions and characterised 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 a dual localization for SAMC1. These findings suggest that the provision of cytosolically synthesized S-adenosylmethionine to mitochondria and possibly also to plastids is mediated by SAMC1 according to the relative demands for this metabolite in the organelles.