The solute carrier (SLC) group of membrane transport proteins include over 400 members organized into 66 families. [1] [2] Most members of the SLC group are located in the cell membrane. The SLC gene nomenclature system was originally proposed by the HUGO Gene Nomenclature Committee (HGNC) and is the basis for the official HGNC names of the genes that encode these transporters. A more general transmembrane transporter classification can be found in TCDB database.
Solutes that are transported by the various SLC group members are extremely diverse and include both charged and uncharged organic molecules as well as inorganic ions and the gas ammonia.
As is typical of integral membrane proteins, SLCs contain a number of hydrophobic transmembrane alpha helices connected to each other by hydrophilic intra- and extra-cellular loops. Depending on the SLC, these transporters are functional as either monomers or obligate homo- or hetero-oligomers. Many SLC families are members of the major facilitator superfamily.
By convention of the nomenclature system, members within an individual SLC family have greater than 20-25% sequence identity to each other. In contrast, the homology between SLC families is very low to non-existent. [3] Hence, the criteria for inclusion of a family into the SLC group is not evolutionary relatedness to other SLC families but rather functional (i.e., an integral membrane protein that transports a solute).
The SLC group include examples of transport proteins that are:
The SLC series does not include members of transport protein families that have previously been classified by other widely accepted nomenclature systems including:
Most members of the SLC group are located in the cell membrane, but some members are located in mitochondria (the most notable one being SLC family 25) or other intracellular organelles.
Names of individual SLC members have the following format: [4]
where:
For example, SLC1A1 is the first isoform of subfamily A of SLC family 1.
An exception occurs with SLC family 21 [5] (the organic anion transporting polypeptide transporters), which for historical reasons have names in the format SLCOnXm where n = family number, X = subfamily letter, and m = member number.
While the HGNC only assign nomenclature to human genes, by convention vertebrate orthologs of these genes adopt the same nomenclature (e.g., VGNC-assigned orthologs of SLC10A1). For rodents, the case of the symbols differs from other vertebrates by using title case, i.e. Slc1a1 denotes the rodent ortholog of the human SLC1A1 gene.
The following families are named under SLC: [6]
Putative SLCs, also called atypical SLCs, are novel, plausible secondary active or facilitative transporter proteins that share ancestral background with the known SLCs. [2] [49] The atypical SLCs of MFS type can, however, be subdivided into 15 Putative MFS Transporter Families (AMTF). [49]
All the putative SLCs are plausible SLC transporters. Some are only "atypical" when it comes to their nomenclature; the genes have an SLC assignment but as an alias, and have retained their already assigned "non-SLC" gene symbol as the approved symbol.
Here are some Putative SLCs listed: OCA2, CLN3, TMEM104, SPNS1, SPNS2, SPNS3, SV2A, SV2B, SV2C, SVOP, SVOPL, MFSD1, [50] MFSD2A, MFSD2B, MFSD3, [50] MFSD4A, [51] MFSD4B, MFSD5, [52] MFSD6, MFSD6L, MFSD8, MFSD9, [51] MFSD10, MFSD11, [52] MFSD12, MFSD13A, MFSD14A, [53] MFSD14B, [53] UNC93A [54] [55] and UNC93B1.
A membrane transport protein is a membrane protein involved in the movement of ions, small molecules, and macromolecules, such as another protein, across a biological membrane. Transport proteins are integral transmembrane proteins; that is they exist permanently within and span the membrane across which they transport substances. The proteins may assist in the movement of substances by facilitated diffusion or active transport. The two main types of proteins involved in such transport are broadly categorized as either channels or carriers. The solute carriers and atypical SLCs are secondary active or facilitative transporters in humans. Collectively membrane transporters and channels are known as the transportome. Transportomes govern cellular influx and efflux of not only ions and nutrients but drugs as well.
Cotransporters are a subcategory of membrane transport proteins (transporters) that couple the favorable movement of one molecule with its concentration gradient and unfavorable movement of another molecule against its concentration gradient. They enable coupled or cotransport and include antiporters and symporters. In general, cotransporters consist of two out of the three classes of integral membrane proteins known as transporters that move molecules and ions across biomembranes. Uniporters are also transporters but move only one type of molecule down its concentration gradient and are not classified as cotransporters.
CD98 is a glycoprotein that is a heterodimer composed of SLC3A2 and SLC7A5 that forms the large neutral amino acid transporter (LAT1). LAT1 is a heterodimeric membrane transport protein that preferentially transports branched-chain and aromatic amino acids. LAT is highly expressed in brain capillaries relative to other tissues.
Sodium-dependent glucose cotransporters are a family of glucose transporter found in the intestinal mucosa (enterocytes) of the small intestine (SGLT1) and the proximal tubule of the nephron. They contribute to renal glucose reabsorption. In the kidneys, 100% of the filtered glucose in the glomerulus has to be reabsorbed along the nephron. If the plasma glucose concentration is too high (hyperglycemia), glucose passes into the urine (glucosuria) because SGLT are saturated with the filtered glucose.
The Na–K–Cl cotransporter (NKCC) is a transport protein that aids in the secondary active transport of sodium, potassium, and chloride into cells. In humans there are two isoforms of this membrane transport protein, NKCC1 and NKCC2, encoded by two different genes. Two isoforms of the NKCC1/Slc12a2 gene result from keeping or skipping exon 21 in the final gene product.
The sodium/phosphate cotransporter is a member of the phosphate:Na+ symporter (PNaS) family within the TOG Superfamily of transport proteins as specified in the Transporter Classification Database (TCDB).
Potassium-dependent sodium-calcium exchanger also known as solute carrier family 24 (SLC24) is a type of sodium-calcium exchanger that requires potassium to function.
Sodium/glucose cotransporter 1 (SGLT1) also known as solute carrier family 5 member 1 is a protein in humans that is encoded by the SLC5A1 gene which encodes the production of the SGLT1 protein to line the absorptive cells in the small intestine and the epithelial cells of the kidney tubules of the nephron for the purpose of glucose uptake into cells. Recently, it has been seen to have functions that can be considered as promising therapeutic target to treat diabetes and obesity. Through the use of the sodium glucose cotransporter 1 protein, cells are able to obtain glucose which is further utilized to make and store energy for the cell.
A neurotransmitter sodium symporter (NSS) (TC# 2.A.22) is type of neurotransmitter transporter that catalyzes the uptake of a variety of neurotransmitters, amino acids, osmolytes and related nitrogenous substances by a solute:Na+ symport mechanism. The NSS family is a member of the APC superfamily. Its constituents have been found in bacteria, archaea and eukaryotes.
An amino acid transporter is a membrane transport protein that transports amino acids. They are mainly of the solute carrier family.
Solute carrier family 13 member 3 also called sodium-dependent dicarboxylate transporter (NaDC3) is a protein that in humans is encoded by the SLC13A3 gene.
Nucleoside transporters (NTs) are a group of membrane transport proteins which transport nucleoside substrates like adenosine across the membranes of cells and/or vesicles. There are two known types of nucleoside transporters, concentrative nucleoside transporters and equilibrative nucleoside transporters, as well as possibly a yet-unidentified vesicular transporter.
Zinc transporter ZIP10, also known as solute carrier family 39 member 10, is a protein that in humans is encoded by the SLC39A10 gene. ZIP10 belongs to a subfamily of proteins that show structural characteristics of zinc transporters, and have 14 members in the human genome: ZIP1, ZIP2, ZIP3, ZIP4, ZIP5, ZIP6, ZIP7, ZIP8, ZIP9, ZIP10, ZIP11, ZIP12, ZIP13 and ZIP14.
Monocarboxylate transporter 10, also known as aromatic amino acid transporter 1 and T-type amino acid transporter 1 (TAT1) and solute carrier family 16 member 10 (SLC16A10), is a protein that in humans is encoded by the SLC16A10 gene. SLC16A10 is a member of the solute carrier family.
Solute carrier family 12 member 8 (SLC12A8), also known as cation-chloride cotransporter 9 (CCC9), is a protein that in humans is encoded by the SLC12A8 gene.
This family of proteins are found both in prokaryotes and eukaryotes. In mammals, they are transmembrane proteins with functions in the liver and in the intestine. They are members of the solute carrier family of cotransporter genes which include SLC10A1 and SLC10A2.
Glucose-6-phosphate exchanger SLC37A2 is a protein that in humans is encoded by the SLC37A2 gene.
Monocarboxylate transporter 3 (MCT3) also known as solute carrier family 16 member 8 is a protein that in humans is encoded by the SLC16A8 gene. MCT is a proton-coupled monocarboxylate transporter. It catalyzes the rapid transport across the plasma membrane of many monocarboxylates such as lactate, pyruvate, branched-chain oxo acids derived from leucine, valine and isoleucine, and the ketone bodies acetoacetate, beta-hydroxybutyrate and acetate. It also functions as high-affinity pyruvate transporter.
The amino acid-polyamine-organocation (APC) superfamily is the second largest superfamily of secondary carrier proteins currently known, and it contains several Solute carriers. Originally, the APC superfamily consisted of subfamilies under the transporter classification number. This superfamily has since been expanded to include eighteen different families.
Glucose-6-phosphate exchanger SLC37A1 is a protein that in humans is encoded by the SLC37A1 gene. SLC37A1 locates to the membrane of the endoplasmic reticulum (ER), and is a glucose 6-phosphate:inorganic phosphate antiporter, transporting glucose 6-phosphate from the cytoplasm into the lumen of the ER, while transporting phosphate in the opposite direction.
SLC Tables. SLCtables