cDNA sequence of a human skeletal muscle ADP/ATP translocator: lack of a leader peptide, divergence from a fibroblast translocator cDNA, and coevolution with mitochondrial DNA genes.

Abstract

We have characterized a 1400-nucleotide cDNA for the human skeletal muscle ADP/ATP translocator. The deduced amino acid sequence is 94% homologous to the beef heart ADP/ATP translocator protein and contains only a single additional amino-terminal methionine. This implies that the human translocator lacks an amino-terminal targeting peptide, a conclusion substantiated by measuring the molecular weight of the protein synthesized in vitro. A 1400-nucleotide transcript encoding the skeletal muscle translocator was detected on blots oftotal RNA from human heart, kidney, skeletal muscle, and HeLa cells by hybridization with oligonucleotide probes homologous to the coding region and 3' noncoding region of the cDNA. However, the level of this mRNA varied substantially among tissues. Comparison of our skeletal muscle translocator sequence with that of a recently published human fibroblast translocator cognate revealed that the two proteins are 88% identical and diverged about 275 million years ago. Hence, tissues vary both in the level of expression of individual translocator genes and in differential expression of cognate translocator genes. Comparison of the base substitution rates of the ADP/ATP translocator and the oxidative phosphorylation genes encoded by mitochondrial DNA revealed that the mitochondrial DNA genes fix 10 times more synonymous substitutions and 12 times more replacement substitutions; yet, these nuclear and cytoplasmic respiration genes experience comparable evolutionary constraints. This suggests that the mitochondrial DNA genes are highly prone to deleterious mutations.The ADP/ATP translocator, or adenine nucleotide translocator (ANT), is the most abundant mitochondrial protein (1). In its functional state it forms a dimer consisting of two identical 30-kDa subunits embedded asymmetrically in the inner mitochondrial membrane (2). The dimer forms a gated pore through which ADP is moved across the inner membrane into the mitochondrial matrix and ATP is moved from the matrix into the cytoplasm (2).Mitochondrial energy production varies greatly in importance between human tissues (3). Because the ANT determines the rate of ADP/ATP flux between the mitochondrion and the cytosol, the ANT would be a logical site for regulating cellular dependence on oxidative energy metabolism. Such regulation could be accomplished by producing varying amounts of the ANT or by elaborating tissue-specific ANT isoforms with different kinetic properties. Although Neurospora crassa has only one ANT gene (4), antigenic and electrophoretic mobility differences among bovine heart, kidney, and liver ANTs (5, 6) suggest that mammals may have multiple ANT genes that are expressed in a tissuespecific manner. Tissue-specific expression of functionally similar genes encoding proteins involved in oxidative phosphorylation (Ox/Phos) has been reported for the bovine ATP synthase proteolipid (7).The ANT and most other Ox/Phos genes are encoded in the nucleus, but 13 essential Ox/Phos polypeptides are encoded in the maternally inh...

Natural selection shaped regional mtDNA variation in humans

“…Both of these analyses revealed a surprising result. ATP6, which is generally found to be one of the more highly conserved genes in comparisons between distant species (25)(26)(27), had the highest amino acid sequence variation of any human mtDNA gene (Fig. 2).…”

“…2). Moreover, ATP8, the companion of ATP6 in the ATP synthase, is one of the least conserved mtDNA protein coding genes in comparisons between distant species (25,26). Yet it proved to be one of the most highly conserved genes in the human mtDNAs (Fig.…”

Bioenergetics, the origins of complexity, and the ascent of man

“…A striking feature of the radiation of mammalian and primate genomic elements is that mtDNA sequences show a much greater sequence evolution rate than do nDNA sequences (13)(14)(15). This rapid mtDNA radiation is reflected in the high degree of functional and regional variation of human mtDNAs (16).…”

Immunological characterization of the mitochondrial 2‐oxoglutarate carrier from liver and heart

“…Tissue-specific differences of mitochondrial aniontransporting systems have so far been demonstrated only for the ADP/ATP carrier by immunological studies [9] as well as cDNA sequence analysis [10][11][12][13][14][15]. It is interesting that in addition to tissue-specific also conformation-specific antibodies could be raised in rabbits against the ADP/ATP carrier [16].…”