Biogenesis and function of the mitochondrial solute carrier (SLC25) family in yeast

The mitochondrial solute carrier family, also called SLC25 family, comprises a group of structurally and evolutionary related transporters that are embedded in the mitochondrial inner membrane. About 35 and 53 mitochondrial carrier proteins are known in yeast and human cells, respectively, which transport nucleotides, metabolites, amino acids, fatty acids, inorganic ions and cofactors across the inner membrane. They are proposed to function by a common rocker-switch mechanism, alternating between conformations that expose substrate-binding pockets to the intermembrane space (cytoplasmic state) and to the matrix (matrix state). The substrate specificities of both states differ so that carriers can operate as antiporters, symporters or uniporters. Carrier proteins share a characteristic structure comprising six transmembrane domains and expose both termini to the intermembrane space. Most carriers lack N-terminal presequences but use carrier-specific internal targeting signals that direct them into mitochondria via a specific import route, known as the 'carrier pathway'. Owing to their hydrophobicity and aggregation-prone nature, the mistargeting of carriers can lead to severe proteotoxic stress and diseases. In this review article, we provide an overview about the structure, biogenesis and physiology of carrier proteins, focusing on baker's yeast where their biology is particularly well characterized.