Complementary DNA Sequence of an Adenylate Translocator from <i>Arabidopsis thaliana</i> (1992)

The adenylate translocator (also called ADP/ATP translocator or carrier) has been intensively studied as a protein of the mitochondrial inner membrane (5, 9). In mitochondria, the ADP/ATP translocator mediates the exchange of mitochondrial ATP, generated by oxidative phosphorylation, with cytosolic ADP in a 1:1 exchange. This exchange process links the energy metabolism of the two different compartments. Multiple nuclear genes have been found to code for the translocator in different organisms: two in maize (10), three in yeast (6), and at least three in humans (4). In plants, it has been shown that an adenylate translocator is also present in the inner membranes of amyloplasts (1, 7) and is able to mediate the uptake of ADP-glucose (8). To study the expression of the gene(s) and the localization of the translocator in different plant organelles, we screened a XgtlO cDNA library from Arabidopsis thaliana (L. Heynh, Columbia strain) using the pANT1 cDNA from maize (10) as a probe. Preliminary screening yielded a partial cDNA of 800 bp encoding for the COOH terminus of a protein homologous to the maize translocator as determined by the analysis of the nucleotide sequence. A second screening, using the 800bp partial clone, yielded a putative full-length cDNA of 1393 bp. This cDNA has been called AANT1 (Table I). The nucleotide sequence of this cDNA shows the occurence of an open reading frame of 1137 nucleotides and a 230nucleotide long noncoding 3' end followed by a polyadenylation stretch of 23 nucleotides (Fig. 1). In the 3'-untranslated region, a putative polyadenylation signal 5'-AATAAA-3' is present, 18 nucleotides upstream from the beginning of the polyadenylation sequence. The open reading frame codes a 379-amino acid polypeptide, as deduced from the nucleotide sequence. This translation product shows 87.0, 75.8, 68.4, and 53.2% identities with the maize (MANT1), yeast (AAC2), Neurospora, and human ADP/ATP translocators, respectively (2, 3, 6, 10). The NH2-terminal amino acid regions differ significantly. Indeed, this region is highly heterologous among the different organisms in terms of length and amino acid sequence but also within one species, Saccharomyces cerevisiae, where the three adenylate translocators so far known do not share the same NH2-terminal region (6). According to the Arabidopsis and maize sequences, it seems that the NH2-terminal region of the adenylate translocator is much longer in the higher plant protein. The comparisons between the mol wts as estimated by immunobloting and the ones deduced from genomic or cDNA sequences did not show significant differences for most of the organisms previously cited. However, in the case of the cDNA that we isolated, the mol wt of the deduced protein is 41,295, compared to the apparent mol wt of 31,000 as determined from immunoblot analysis of partially purified proteins of Arabidopsis leaves (data not shown). To account for this discrepancy, one might propose that translation proceeds from another initiation site located further downstream from the first ATG codon. Another possibility may be that a larger precursor protein is made, which undergoes posttranslational processing to yield a smaller mature protein. In such a case, the plant system appears to be different from other reported systems. Further experiments are currently in progress to confirm the length of the full mRNA and to distinguish between these two alternatives.