Abstract
Although structurally related, mitochondrial carrier family (MCF) proteins catalyze the specific transport of a range of diverse substrates including nucleotides, amino acids, dicarboxylates, tricarboxylates, cofactors, vitamins, phosphate and H+. Despite their name, they do not, however, always localize to the mitochondria, with plasma membrane, peroxisomal, chloroplast and thylakoid and endoplasmic reticulum localizations also being reported. The existence of plastid-specific MCF proteins is suggestive that the evolution of these proteins occurred after the separation of the green lineage. That said, plant-specific MCF proteins are not all plastid-localized, with members also situated at the endoplasmic reticulum and plasma membrane. While by no means yet comprehensive, the in vivo function of a wide range of these transporters is carried out here, and we discuss the employment of genetic variants of the MCF as a means to provide insight into their in vivo function complementary to that obtained from studies following their reconstitution into liposomes.
Highlights
The characterization of heterologously expressed, liposome-reconstituted proteins has provided considerable insight into the transport capacities of human, yeast and plant mitochondrial carrier family (MCF) members [1,2,3,4,5,6,7,8,9,10,11,12,13]
Plants produce coenzyme A (CoA) in the cytosol which is than imported into the organelles where it is used in essential pathways such as the tricarboxylic acid (TCA) cycle in the mitochondria, fatty acid synthesis in the chloroplast, and β-oxidation in peroxisomes
It was suggested that AtNDT2 protein might be a key regulator of the mitochondrial NAD+ and NADH pools and compromised NAD+ import activity in ndt2 mutants cannot be fully compensated for by other transporters [87], highlighting the importance role of NDT2 for NAD+ import by plant mitochondria. These results suggest that correct NDT1 and NDT2 expression is necessary for maintaining NAD+ balance among organelles that modulate metabolism, physiology and developmental processes in plant tissues
Summary
The characterization of heterologously expressed, liposome-reconstituted proteins has provided considerable insight into the transport capacities of human, yeast and plant MCF members [1,2,3,4,5,6,7,8,9,10,11,12,13]. Inference from phylogenetic analysis has been demonstrated, at least in some cases, to provide good hints as to the function of plant MCF proteins on the basis of the experimentally characterized functions of, for example, yeast proteins with which they share high homology [14] While these studies have provided a wealth of information concerning the potential substrates that can be transported by a relatively large number of the transporters [15], the biological relevance of these properties remains dependent on the context of where and when they are expressed. The presence of a subfamily in a single lineage, for example the mitochondrial GTP/GDP transporter of yeast or the plastidial adenine nucleotide carriers and Brittle protein, is likely indicative of an innovation that occurred after the separation of the eukaryotes. We will summarize functional insights obtained from evaluating spatio-temporal differences in expression, subcellular localization and from the study of transgenic plants exhibiting altered expression of the transporter(s) of interest
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