Membrane transporters are the gatekeepers for all cells and organelles, controlling uptake and efflux of crucial compounds such as sugars, nucleotides, inorganic ions, and drugs (Hediger et al. 2004). They are responsible for substrate movement across both cytoplasmic membranes of cells and internal membranes of organelles (Sreedharan et al. 2011). Transporters can be divided into ABC transporters, pumps, ion channels, water channels, and solute carriers. Membrane bound proteins represent about 27% of the entire human proteome. Among the membrane bound proteins, the SLC transporters are the second largest group after G protein coupled receptors (Lagerstrom 2008Almen et al. 2009). Transporters can be divided into two families, passive and active transporters. The active transporters use diverse energy-coupling mechanisms to allow the movement of molecules across a membrane against a concentration gradient. The passive transporters, also known as facilitated transporters, allow passage of solutes (e.g., glucose, amino acids, urea) across membranes down their electrochemical gradients (Hediger et al. 2004).
Appreciation of the role that transport proteins play in the absorption, distribution, and elimination of a wide variety of drugs in clinical use is increasing. As the largest group of secondary transporters, SLC transporters are becoming the focus of an increasing number of studies because they control transmembrane movement of many types of important substrates. The human genome contains approximately 360 unique SLC protein genes grouped into 48 families (Ren et al. 2007; Fredriksson et al. 2008). Approximately 19 of the SLC gene families have been reported to transport xenobiotics including: organic anion polypeptides (SLCO), oligopeptides (SLC15) (Russel et al. 2002; Brandsch et al. 2008; Dobson and Kell 2008; Rubio and Daniel 2008), organic anion/cations (SLC22) (Koepsell et al. 2007; Ciariboli 2008), and organic cations (SLC47) (Tanihara et al. 2007; Moriyama et al. 2008; Matsushima et al. 2009).
The SLC25 gene encodes mitochondrial carriers (MCs), which are membrane-integrated proteins that localize to the inner membranes of mitochondria and catalyze the translocation of solutes across the membranes (Plamieri, 2004). The MCs provide a critical link between the mitochondria and the cytosol by facilitating the flux of solutes through the permeable barrier of the inner mitochondrial membrane. The substrates transported by the MCs range from the smallest H+ to the largest ATP molecule, implying that they have a broad array of functions in diverse metabolic processes. Defects in MC genes lead to several diseases such as type II citrullinaemia (SLC25A13; OMIM 215700), hyperornithine-hyperammone-homocitrulline-mia (HHH) syndrome (SLC25A15; OMIM 238970), Stanley syndrome (SLC25A20; OMIM 212138), Amish microcephaly (SLC25A20; OMIM 607196), and autosomal dominant progressive external ophthalmoplegia (adPEO) (SLC25A4; OMIM 157640). The complete amino acid sequence of the ATP/ADP carrier was identified in beef heart mitochondria (Aquila et al. 1982; Aquila et al. 1985).
Post-genomic era studies have enabled us to identify many more mitochondria carrier families (MCFs) simultaneously without laborious cloning or purification procedures. Although much is known about the characteristics and functions of MCFs in human and plants, their biological roles in fish remain unknown. In our studies, we cloned the CcSLC25a5 (Cyprinus Carpio SLC25a5) gene using Genefishing kits from the skins of the mirror carp, which has interspersed scales, and the Jianli, that has full scales. The expression pattern of SLC25a5 during different developmental stages was determined by whole-mount in situ hybridization.