Mitochondrial carrier family: repertoire and peculiarities of the cellular slime mould Dictyostelium discoideum.

Proteins of the mitochondrial carrier family (MCF) mediate the transport of a large range of compounds, including metabolites and cofactors. They are localized mainly in the inner mitochondrial membrane, except for a few members found in the membranes of peroxisomes. Similarity searches among Dictyostelium discoideum protein sequences identified a total of 31 MCF members. All these are membrane proteins that possess three characteristic repeats of a domain of approximately 100 residues. Among them, three proteins have supplementary structural domains consisting of Ca(2+)-binding motifs made up of 2 or 4 EF-hand units localized on the N-terminal end, facing the mitochondrial intermembrane space. The nature of transported substrates is proposed on the basis of sequence comparison with orthologs characterized biochemically in other organisms, of phylogenetic analysis, and of the conservation of discriminating amino acid residues belonging to the substrate binding sites. Carriers have been grouped in subclasses based on their specificity for the transport of nucleotides, amino acids or keto acids. Furthermore, we have identified an iron carrier of the mitoferrin type, an inorganic phosphate carrier, and three carriers with similarity to uncoupler proteins. This study provides a focus for mitochondrial carrier analysis in Dictyostelium discoideum.