Molecular identification of an Arabidopsis S-adenosylmethionine transporter. Analysis of organ distribution, bacterial expression, reconstitution into liposomes, and functional characterization.
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.
Palmieri
Luigi
L
Department of Pharmaco-Biology, Laboratory of Biochemistry and Molecular Biology, University of Bari, 70125 Bari, Italy.
Arrigoni
Roberto
R
Blanco
Emanuela
E
Carrari
Fernando
F
Zanor
Maria Ines
MI
Studart-Guimaraes
Claudia
C
Fernie
Alisdair R
AR
Palmieri
Ferdinando
F
D000595:Amino Acid Sequence; D017360:Arabidopsis / Q000235:genetics / Q000378:metabolism*; D029681:Arabidopsis Proteins / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism*; D004926:Escherichia coli / Q000235:genetics*; D015870:Gene Expression; D018506:Gene Expression Regulation, Plant; D008081:Liposomes / Q000737:chemistry / Q000378:metabolism*; D026901:Membrane Transport Proteins / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism*; D008969:Molecular Sequence Data; D021381:Protein Transport; D012436:S-Adenosylmethionine / Q000378:metabolism*; D013379:Substrate Specificity
D029681:Arabidopsis Proteins; D008081:Liposomes; D026901:Membrane Transport Proteins; C516463:SAMC1 protein, Arabidopsis; C516464:SAMC2 protein, Arabidopsis; D012436:S-Adenosylmethionine