About this Research Topic
In this research topic we will present a survey about the current knowledge on solute transport proteins with special focus on the mitochondrial carrier family (MCF). We want to introduce the reader into the fascinating and sometimes very surprising results obtained from analyses concerning membrane proteins and transport processes.
The MCF is a large transporter family and members (one to more than 50) can be found in nearly all eukaryotic organisms from protists to higher plants and humans, whereas such proteins have never been detected in prokaryotic species. This observation supports the hypothesis that this prominent transporter family was newly invented slightly before or during establishment of the mitochondrial ancestor in the first eukaryotic cell. Various phylogenetically basal protists with rather ancient mitochondria-related organelles or mitochondrial derivatives (so called hydrogenosomes, mitosomes or anaerobic mitochondria) were shown to harbor MCF proteins. A common ancestral origin of recent MCF members is substantiated by phylogenetic analyses and reflected by their common basic structure. Generally, MCF proteins exhibit a molecular mass of about 30 – 35 kDa, they consist of six transmembrane domains composed of three repetitive regions with conserved amino acid motifs. Accordingly, each repeated domain comprises two transmembrane alpha-helices. Moreover, each loop connecting one odd-numbered with the following even-numbered helix exhibits a short amphipathic helical domain. Although MCF proteins are quite similar in structure, they are very heterogenous in terms of their biochemical properties and physiological function.
MCF proteins mediate the transport of nearly all solutes shuttled across the inner mitochondrial membrane and represent carriers specific for diverse nucleotides, basic amino acids, di- and tricarboxylates, phosphate, for a broad variety of cofactors as well as uncoupling proteins, etc. The name might imply that MCF proteins are restricted to mitochondria (and mitochondria-related organelles). That, however, is not the whole story. Even though most MCF proteins are located in the inner mitochondrial membrane, recent studies detected more and more members also in other organelles, like the endoplasmic reticulum, peroxisomes and chloroplasts or in the plasma membrane. The functional properties of MCF proteins in other compartments than mitochondria appear to be more restricted because so far only carriers for adenine nucleotides or adenylated cofactors have been identified. Solute transport at the inner mitochondrial membrane is basically dominated by MCF proteins whereas solute transport of plastids, vacuoles, the plasma membrane, ER, or Golgi apparatus involves proteins from diverse families.
Here we present an overview about structural and functional characteristics of solute transport proteins from different compartments and organisms. We highlight important key methods for investigation of membrane proteins and particularly refer to techniques applied for the analysis of solute transport. Diverse experts with long-term and short-term experience in membrane protein characterization summarize their scientific findings, share positive impressions and specify possible difficulties that might occur in transport protein analysis.
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