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Introduction to Biological Membranes
Biological membranes are composed of lipid, protein and carbohydrate that exist in a fluid state. Biological membranes are the structures that define and control the composition of the space that they enclose. All membranes exist as dynamic structures whose composition changes throughout the life of a cell. In addition to the outer membrane that results in the formation of a typical cell (this membrane is often referred to as the plasma membrane), cells contain intracellular membranes that serve distinct functions in the formation of the various intracellular organelles, e.g. the nucleus and the mitochondria.
back to the topComposition and Structure of Biological Membranes
As indicated above, biological membranes are composed of lipids, proteins, and carbohydrates. The carbohydrates of membranes are attached either to lipid forming glycolipids of various classes, or to proteins forming glycoproteins. The lipid and protein compositions of membranes vary from cell type to cell type as well as within the various intracellular compartments that are defined by intracellular membranes. Protein concentrations can range from around 20% to as much as 70% of the total mass of a particular membrane.
The lipids making up components of membranes are of three major classes that includes glycerophospholipids, sphingolipids, and cholesterol. For information on the structures of these different lipid classes see the Lipids page, Lipid Synthesis page, Sphingolipids page and the Cholesterol page. Sphingolipids and glycerophospholipids constitute the largest percentage of the lipid weight of biological membranes. The hydrocarbon tails of these two classes of lipid result in steric limitations to their packing such that they will form disk-like micelles. The structure of these micelles results from the interactions of the hydrophobic tails of the lipids and the exposure of the polar head groups to the aqueous environment. This orientation results in what is referred to as a lipid bilayer and is diagrammed in the figure below. Lipid bilayers are essentially two-dimensional fluids and the lipid components of the bilayer can diffuse laterally and in fact evidence demonstrates that this lateral diffusion occurs readily. Lipids in the bilayer can also undergo transverse diffusion (also called a flip-flop) where the lipid diffuses from one surface to the other. However, because the flip-flop requires the polar head group to pass through the hydrocarbon core of the bilayer the process is extremely rare. Enzymes have been identified that facilitate the flip-flop process and these enzymes are referred to as flipases.
Structure of a typical lipid bilayer
Biological membranes also contain proteins, glycoproteins, and lipoproteins (see the Glycoproteins and Protein Modifications pages). Proteins associated with membranes are of two general types: integral and peripheral. Integral membrane proteins (also called intrinsic proteins) are tightly bound to the membrane through hydrophobic interactions and are inserted into and/or penetrate the lipid bilayer. In contrast, peripheral membrane proteins (also called extrinsic proteins) are only loosely associated with the membrane either through interactions with the polar head groups of the lipids or through interactions with integral membrane proteins. Peripheral membrane proteins are most often, if not exclusively, found on the cytosolic face of the plasma membrane or the lumenal surface of subcellular organelle membranes.
Proteins that are found associated with membranes can also be modified by lipid attachment (lipoproteins). The lipid portion of a lipoprotein anchors the protein to the membrane either through interaction with the lipid bilayer directly or through interactions with integral membrane proteins. Lipoproteins associated with membranes contain one of three types of covalent lipid attachment. The lipids are isoprenoids such as farnesyl and geranylgeranyl residues (see the Protein Modifications page for the mechanism of protein prenylation), fatty acids such as myristic and palmitic acid, and glycosylphosphatidylinositol, GPI (termed glipiated proteins: see the Glycoproteins page for details).
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Activities of Biological Membranes
Although biological membranes contain various types of lipids and proteins, their distribution between the two different sides of the bilayer is asymmetric. As a general example the outer surface of the bilayer is enriched in phosphatidylethanolamine, whereas the intracellular surface is enriched in phosphatidylcholine. Carbohydrates, whether attached to lipid or protein, are almost exclusively found on the external surfaces of membranes. The asymmetric distribution of lipids and proteins in membranes results in the generation of highly specialized sub-domains within membranes. In addition, there are highly specialized membrane structures such as the endoplasmic reticulum (ER), the Golgi apparatus and vesicles. The most important vesicles are those that contain secreted factors. Membrane bound proteins (e.g. growth factor receptors) are processed as they transit through the ER to the Golgi apparatus and finally to the plasma membrane. As these proteins transit to the surface of the cell they undergo a series of processing events that includes glycosylation.
The vesicles that pinch off from the Golgi apparatus are termed coated vesicles. The membranes of coated vesicles are surrounded by specialized scaffolding proteins that will interact with the extracellular environment. There are three major types of coated vesicles that are characterized by their protein coats. Clathrin-coated vesicles contain the protein clathrin and are involved in transmembrane protein, GPI-linked protein and secreted protein transit to the plasma membrane. COPI (COP = coat protein) forms the surface of vesicles involved in the transfer of proteins between successive Golgi compartments. COPII forms the surface of vesicles that transfer proteins from the ER to the Golgi apparatus. Clathrin-coated vesicles are also involved in the process of endocytosis such as occurs when the LDL receptor binds plasma LDLs for uptake by the liver. The membrane location of these types of receptors is called a clathrin-coated pit.
In addition, certain cells have membrane compositions that are unique to one surface of the cell versus the other. For instance epithelial cells have a membrane surface that interacts with the lumenal cavity of the organ and another that interacts with the surrounding cells. The membrane surface of cells that interacts with lumenal contents is referred to as the apical surface or domain, the rest of the membrane is referred to as the basolateral surface or domain. The apical and basolateral domains do not intermix and contain different compositions of lipid and protein.
Most eukaryotic cells are in contact with their neighboring cells and these interactions are the basis of the formation of organs. Cells that abut one another are in metabolic contact which is brought about by specialized tubular particles called gap junctions. Gap junctions are intercellular channels and their presence allows whole organs to be continuous from within. One major function of gap junctions is to ensure a supply of nutrients to cells of an organ that are not in direct contact with the blood supply. Gap junctions are formed from a type of protein called a connexin.
Given the predominant lipid nature of biological membranes many types of molecules are restricted in their ability to diffuse across a membrane. This is especially true for charged ions, water and hydrophilic compounds. The barrier to membrane translocation is overcome by the presence of specialized channels and transporters. Although channels and transporters are required to move many types of molecules and compounds across membranes, some substances can pass through from one side of a membrane to the other through a process of diffusion. Diffusion of gases such as O2, CO2, NO, and CO occurs at a rate that is solely dependent upon concentration gradients. Lipophilic molecules will also diffuse across membranes at a rate that is directly proportional to the solubility of the compound in the membrane. Although water can diffuse across biological membranes, the physiological need for rapid equilibrium across plasma membranes has lead to the evolution of a family of water transporting channels that are called aquaporins (see section below).
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Membrane Channels
The definition of a channel (or a pore) is that of a protein structure that facilitates the translocation of molecules or ions across the membrane through the creation of a central aqueous channel in the protein. This central channel facilitates diffusion in both directions dependent upon the direction of the concentration gradient. Channel proteins do not bind or sequester the molecule or ion that is moving through the channel. Specificity of channels for ions or molecules is a function of the size and charge of the substance. The flow of molecules through a channel can be regulated by various mechanisms that result in opening or closing of the passageway.
Membrane channels are of three distinct types. The α-type channels are homo- or hetero-oligomeric structures that in the latter case consist of several dissimilar proteins. This class of channel protein has between 2 and 22 transmembrane α-helical domains which explains the derivation of their class. Molecules move through α-type channels down their concentration gradients and thus require no input of metabolic energy. Some channels of this class are highly specific with respect to the molecule translocated across the membrane while others are not. In addition, there may be differences from tissue to tissue in the channel used to transport the same molecule. As an example, there are over 15 different K+-specific voltage-regulated channels in humans.
The transport of molecules through α-type channels occurs by several different mechanisms. These mechanisms include changes in membrane potential (termed voltage-regulated or voltage-gated), phosphorylation of the channel protein, intracellular Ca2+, G-proteins, and organic modulators.
Aquaporins (AQP) are a family of α-type channels responsible for the transport of water across membranes. At least 11 aquaporin proteins have been identified in mammals with 10 known in humans (termed AQP0 through AQP9). A related family of proteins is called the aquaglyceroporins which are involved in water transport as well as the transport of other small molecules. AQP9 is the human aquaglyceroporin. The aquaporins assemble in the membrane as homotetramers with each monomer consisting of six transmembrane α-helical domains forming the distinct water pore. Probably the most significant location of aquaporin expression is in the kidney. The proximal tubule expresses AQP1, AQP7, and AQP8, while the collecting ducts express AQP2, AQP3, AQP4, AQP6, and AQP8. Loss of function of the renal aquaporins is associated with several disease states. Reduced expression of AQP2 is associated with nephrogenic diabetes insipidus (NDI), acquired hypokalemia, and hypercalcemia.
The β-barrel channels (also called porins) are so named because they have a transmembrane domain that consists of β-strands forming a β-barrel structure. Porins are found in the outer membranes of mitochondria. The mitochondrial porins are voltage-gated anion channels that are involved in mitochondrial homeostasis and apoptosis.
The pore-forming toxins represent the third class of membrane channels. Although this is a large class of proteins first identified in bacteria, there are a few proteins of this class expressed in mammalian cells. The defensins are a family of small cysteine-rich antibiotic proteins that are pore-forming channels found in epithelial and hematopoietic cells. The defensins are involved in host defense against microbes (hence the derivation of their name) and may be involved in endocrine regulation during infection.
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Membrane Transporters
Transporters are distinguished from channels because they catalyze (mediate) the movement of ions and molecules by physically binding to and moving the substance across the membrane. Transporter activity can be measured by the same kinetic parameters applied to the study of enzyme kinetics. Transporters exhibit specificity for the molecule being transported as well as show defined kinetics in the transport process. Transporters can also be affected by both competitive and noncompetitive inhibitors. Transporters are also known as carriers, permeases, translocators, translocases, and porters. Mediated transporters are classified based upon the stoichiometry of the transport process. Uniporters transport a single molecule at a time, symporters simultaneously transport two different molecules in the same direction, and antiporters transport two different molecules in opposite directions.
The action of transporters is divided into two classifications: passive-mediated transport (also called facilitated diffusion) and active transport. Facilitated diffusion involves the transport of specific molecules from an area of high concentration to one of low concentration which results in an equilibration of the concentration gradient. Glucose transporters are a good example of passive-mediated (facilitative diffusion) transporters. More information on the different glucose/hexose transporters can be found in the Glycolysis page. Another important class of passive-mediated transporters are the K+ channels (see section above).
In contrast, active transporters transport specific molecules from an area of low concentration to that of high concentration. Because this process is thermodynamically unfavorable, the process must be coupled an exergonic process, e.g. hydrolysis of ATP. There are many different classes of transporters that couple the hydrolysis of ATP to the transport of specific molecules. In general these transporters are referred to as ATPases. There are four different types of ATPases, three that transport cations and one that transports anions. The A-type ATPases transport anions. The P-type ATPases are mostly found in the plasma membrane and are involved in the transport of H+, K+, Na+, Ca2+, Cd2+, Cu2+, and Mg2+. These ATPases are so named because they are autophosphorylated by ATP during the transport process. F-type ATPases function in the translocation of H+ in the mitochondria during the process of oxidative phosphorylation. V-type ATPases are located in acidic vesicles and lysosomes and have homology to the F-type ATPases.
One of the most thoroughly studied ATPases is the (Na+-K+)-ATPase found in plasma membranes. This transporter, sometimes called the (Na+-K+)-pump, is involved in the transport of Na+ out of and K+ into cells. The extrusion of Na+ allows cells to control their water content. In addition, the fact that three moles of Na+ are transported out and only two moles of K+ is transporter into the cell, an electrochemical gradient is established. This is the basis for the electrochemical excitability of nerve cells. In fact, it is this transporter action that is the major requirement for ATP production from glucose oxidation in the central nervous system.
back to the topThe ABC Family of Transporters
The ABC transporters comprise the ATP-binding cassette transporter superfamily. All members of this superfamily of membrane proteins contain a conserved ATP-binding domain and use the energy of ATP hydrolysis to drive the transport of various molecules across all cell membranes. There are 48 known members of the ABC transporter superfamily and they are divided into seven subfamilies based upon phylogenetic analyses. These seven subfamilies are designated ABCA through ABCG. Each member of a given subfamily is distinguished with numbers (e.g. ABCA1). In order to keep the size of the following Table limited, only those ABC transporters (of the 48 known genes) whose functions have been defined or assessed by in vitro assays are included.
| Gene Symbol | Other Names | Chromosome | Functions/Comments |
| ABCA1 | ABC1 | 9q31.1 | transfer of cellular cholesterol to HDLs, defects in gene associated with development of Tangier disease |
| ABCA2 | ABC2 | 9q34 | role in delivery of LDL-derived free cholesterol to the endoplasmic reticulum for esterification, involved in protection against reactive oxygen species, drug resistance |
| ABCA4 | ABCR | 1p22.1–p22 | expressed exclusively in retinal photoreceptors, efflux of all trans-retinal aldehyde |
| ABCB1 | PGY1, MDR1 | 7p21.1 | PGY1=P-glycoprotein 1; MDR1=multidrug resistance protein 1, multidrug resistance P-glycoprotein, is an integral component of the blood-brain barrier, transports a number of drugs from the brain back into the blood |
| ABCB2 | TAP1, PSF1, APT1 | 6p21.3 | TAP1=transporter, ATP-binding cassette, major histocompatibility complex (MHC), 1; PSF1=peptide supply factor 1; APT1=antigen peptide transporter1; peptide transport from cytosol to MHC class I molecules in the ER; functions as a heterodimer with ABCB3/TAP2 |
| ABCB3 | TAP2, PSF2, APT2 | 6p21 | TAP2=transporter, ATP-binding cassette, major histocompatibility complex (MHC), 2; PSF2=peptide supply factor 2; APT2=antigen peptide transporter1; peptide transport from cytosol to MHC class I molecules in the ER; functions as a heterodimer with ABCB2/TAP1 |
| ABCB4 | PGY3, MDR3 | 77q21.1 | PGY3=P-glycoprotein 3; MDR3=multidrug resistance protein 3, class III multidrug resistance P-glycoprotein, canalicular phospholipid translocator, biliary phosphatidylcholine transport, defects in gene associated with 6 liver diseases: progressive familial intrahepatic cholestasis type 3 (PFIC3), adult biliary cirrhosis, transient neonatal cholestasis, drug-induced cholestasis, intrahepatic cholestasis of pregnancy, and low phospholipid-associated cholelithiasis syndrome |
| ABCB6 | MTABC3 | 1q42 | mitochondrial transporter involved in heme biosynthesis, transports porphyrins into mitochondria |
| ABCB7 | ABC7 | Xq12–q13 | iron-sulfur (Fe/S) cluster transport |
| ABCB11 | BSEP, SPGP | 2q24 | BSEP=bile salt export protein, bile salt transport out of hepatocytes, gene defects associated with progressive familial intrahepatic cholestasis type 2 (PFIC2) |
| ABCC1 | MRP1 | 16p13.1 | MRP1=multidrug resistance associated protein 1, sphingosine-1-phosphate (S1P) release from mast cells which enhances their migration, uses glutathione as a co-factor in mediating resistance to heavy metal oxyanions |
| ABCC2 | MRP2, CMOAT | 10q24 | MRP2=multidrug resistance associated protein 2, CMOAT=canalicular multispecific organic anion transporter, biliary excretion of many non-bile organic anions, gene defects result in Dubin-Johnson syndrome |
| ABCC3 | MRP3, CMOAT3 | 17q21.3 | MRP3=multidrug resistance associated protein 3, CMOAT3=canalicular multispecific organic anion transporter, drug resistance |
| ABCC4 | MRP4, MOATB | 13q32 | MRP4=multidrug resistance associated protein 4, MOATB=multispecific organic anion transporter B, enriched in prostate, regulator of intracellular cyclic nucleotide levels, mediator of cAMP-dependent signal transduction to the nucleus |
| ABCC5 | MRP5, MOATC | 3q27 | MRP5=multidrug resistance associated protein 5, MOATB=multispecific organic anion transporter C, resistance to thiopurines and antiretroviral nucleoside analogs |
| CFTR | ABCC7 | 7q31.2 | CFTR=cystic fibrosis transmembrane conductance regulator, chloride ion channel, gene defects result in cyctic fibrosis |
| ABCC8 | SUR | 11p15.1 | SUR=sulfonylurea receptor, target of the type 2 diabetes drugs such as glipizide |
| ABCD1 | ALD | Xq28 | involved in the import and/or anchoring of very long-chain fatty acid-CoA synthetase (VLCFA-CoA synthetase) to the peroxisomes, gene defects result in X-linked adrenoleukodystrophy (XALD) |
| ABCE1 | OABP, RNS4I | 4q31 | OABP=oligoadenylate binding protein; RNS4I=Ribonuclease 4 inhibitor |
| ABCG1 | ABC8, White1 | 21q22.3 | involved in mobilization and efflux of intracellular cholesterol, responsible for approximately 20% of cholesterol efflux to HDLs (reverse cholesterol transport) |
| ABCG2 | ABCP, MXR, BCRP | 4q22 | ABCP=ATP-binding cassette transporter, placenta-specific; MXR=mitoxantrone-resistance protein; BCRP=beast cancer resistance protein, xenobiotic transporter, plays a major role in multidrug resistance, heme and porphyrin export |
| ABCG4 | White2 | 11q23.3 | expression restricted to astrocytes and neurons, cholesterol and sterol efflux to HDL-like particles in the CNS, may function in sterol transport with ABCG1 in cells where the two genes are co-expressed, may increase lipidation of apoE in Alzheimer disease |
| ABCG5 | White3 | 2p21 | forms an obligate heterodimer with ABCG8, expressed in intestinal enterocytes and hepatocytes, functions to limit plant sterol and cholesterol absorption from the diet by facilitating efflux out of enterocytes into the intestinal lumen and out of hepatocytes into the bile |
| ABCG8 | Sterolin 2 | 2p21 | see above for ABCG5 |
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The Solute Carrier Family of Transporters
The solute carrier (SLC) family of transporters includes over 300 proteins functionally grouped into 47 families. The SLC family of transporters includes facilitative transporters, primary and secondary active transporters, ion channels, and the aquaporins. The aquaporins are so named because they constitute water channels (see above). Given the scope of this discussion it is not possible to cover all of the transporters in each of the 47 families. Listed below are several of the families of SLC transporters and within each family is a description of several member proteins. All of the members of a particular family are not included due to space limitations. Focus is primarily on solute carriers discussed on other web pages in this site or due to known clinical significance.
| SLC Family | Functional Class | Member Names / Comments |
| 1 | high affinity glutamate and neutral amino acid transporters |
SLC1A1, SLC1A2, SLC1A3, SLC1A4, SLC1A5, SLC1A6, SLC1A7 SLC1A4 and SLC1A5 are the neutral amino acid transporters decreased expression of SLC1A2 is associated with amyotrophic lateral sclerosis (ALS = Lou Gehrig's disease) |
| 2 | facilitative GLUT transporters |
SLC2A1, SLC2A2, SLC2A3, SLC2A4, SLC2A5, SLC2A6, SLC2A7, SLC2A8, SLC2A9, SLC2A10, SLC2A11, SLC2A12, SLC2A13, SLC2A14
SLC2A1 is GLUT1. This glucose transporter is ubiquitously expressed in various tissues but only at low levels in liver and skeletal muscle. This is the primary glucose transporter in erythrocytes. SCL2A2 is GLUT2. This glucose transporter is expressed predominantly in the liver, pancreatic β-cells, kidney, and intestines. SCL2A3 is GLUT3. This glucose transporter is found primarily in neurons and possess the lowest Km for glucose of any of the glucose transporters. SLC2A4 is GLUT4. This glucose transporter is expressed predominantly in insulin-responsive tissues such as skeletal muscle and adipose tissue. SLC2A5 is GLUT5 which is now known to be involved in fructose transport not glucose transport. SCL2A13 is also called the proton (H+) myo–inositol cotransporter, HMIT |
| 3 | heavy subunits of heteromeric amino acid transport | SLC3A1, SLC3A2 |
| 4 | bicarbonate transporters |
SLC4A1, SLC4A2, SLC4A3, SLC4A4, SLC4A5, SLC4A7, SLC4A8, SLC4A9, SLC4A10, SLC4A11 SLC4A7 was formerly identified as SLC4A6 and so the SLC4A6 identity is no longer used |
| 5 | sodium glucose co–transporters | SLC5A1, SLC5A2, SLC5A3, SLC5A4, SLC5A5, SLC5A6, SLC5A7, SLC5A8, SLC5A9, SLC5A10, SLC5A11, SLC5A12 |
| 6 | sodium– and chloride–dependent neurotransmitter transporters |
SLC6A1, SLC6A2, SLC6A3, SLC6A4, SLC6A5, SLC6A6, SLC6A7, SLC6A8, SLC6A9,
SLC6A10, SLC6A11, SLC6A12, SLC6A13, SLC6A14, SLC6A15, SLC6A16, SLC6A17, SLC6A18, SLC6A19, SLC6A20
SLC6A19 is involved in neutral amino acid transport, deficiency results in Hartnup disorder; protein also called system B(0) neutral amino acid transporter 1 [B(0)AT1] |
| 7 | cationic amino acid transporters and the glycoprotein-associated amino acid transporters | SLC7A1, SLC7A2, SLC7A3, SLC7A4, SLC7A5, SLC7A6, SLC7A7, SLC7A8, SLC7A9, SLC7A10, SLC7A11 |
| 8 | Na+/Ca2+ exchangers (NCK proteins) | SLC8A1, SLC8A2, SLC8A3 |
| 9 | Na+/H+ exchangers | SLC9A1, SLC9A2, SLC9A3, SLC9A4, SLC9A5, SLC9A6, SLC9A7, SLC9A8, SLC9A9, SLC9A10 |
| 10 | sodium bile salt cotransporters |
SLC10A1, SLC10A2, SLC10A3, SLC10A4, SLC10A5, SLC10A7
SLC10A1: also called NTCP for Na+-taurocholate cotransporting polypeptide, NTCP is involved in hepatic uptake of bile acids through the sinusoidal/basolateral membrane SLC10A3, SLC10A4, and SLC10A5 are considered orphan transporters |
| 11 | proton-coupled metal ion transporters |
SLC11A1, SLC11A2, SLC11A3
SLC11A2 is also known as the divalent metal-ion transporter-1 (DMT1) SLC11A3 is now referred to as SLC40A1, this protein is more commonly referred to as ferroportin, but is also known as iron-regulated gene 1 (IREG1), and reticuloendothelial iron transporter (MPT1) |
| 12 | electroneutral cation/Cl– cotransporter | SLC12A1, SLC12A2, SLC12A3, SLC12A4, SLC12A5, SLC12A6, SLC12A7, SLC12A8, SLC12A9 |
| 13 | Na+–sulfate/carboxylate cotransporters | SLC13A1, SLC13A2, SLC13A3, SLC13A4, SLC13A5 |
| 14 | urea transporters | SLC14A1, SLC14A2 |
| 15 | proton oligopeptide cotransporters | SLC15A1, SLC15A2, SLC15A3, SLC15A4 |
| 16 | monocarboxylate transporters | SLC16A1, SLC16A2, SLC16A3, SLC16A4, SLC16A5, SLC16A6, SLC16A7, SLC16A8, SLC16A9, SLC16A10, SLC16A11, SLC16A12, SLC16A13, SLC16A14 |
| 17 | organic anion transporters; originally identified as type I Na+–phosphate cotransporters | SLC17A1, SLC17A2, SLC17A3, SLC17A4, SLC17A5, SLC17A6, SLC17A7, SLC17A8, SLC17A9 |
| 18 | vesicular amine transporters | SLC18A1, SLC18A2, SLC18A3 |
| 19 | folate/thiamine transporters | SLC19A1, SLC19A2, SLC19A3 |
| 20 | type III Na+–phosphate cotransporters |
SLC20A1, SLC20A2 also called Pit-1 and Pit-2 (Pi=inorganic phosphate, t=transporter) |
|
21 SLCO |
organic anion transporting polypeptides (OATP) |
there are at least 11 human SLCO family members divided into 6 subfamilies identified as
1 through 6 these transporters have the nomenclature SLCO followed by the family number, subfamily letter, and member number; e.g. SLCO1B1 is a sinusoidal/basolateral membrane Na+-independent transporter, also called the organic anion transporting polypeptide 1B1 (OATP1B1). SLCO1B1 was formerly identified as OATPC and also as SLC21A6. |
| 22 | organic cation transporters (OCTs), zwitterion/cation transporters (OCTNs) and organic anion transporters (OATs) |
SLC22A1, SLC22A2, SLC22A3, SLC22A4, SLC22A5, SLC22A6, SLC22A7, SLC22A8, SLC22A9, SLC22A10,
SLC22A11, SLC22A12, SLC22A15, SLC22A16, SLC22A17, SLC22A18, SLC22A20
SLC22A18: found in the imprinted region of chromosome 11 associated with Beckwith-Wiedemann syndrome (BWS) |
| 23 | Na+–dependent ascorbic acid transporters |
SLC23A1, SLC23A2, SLC23A3, SLC23A4 also identified as SVCT1, SVCT2, SVCT3, and SVCT4 SLC23A3 and SLC23A4 are orphan transporters |
| 24 | Na+/Ca2+–K+ exchangers (NCKX proteins) | SLC24A1, SLC24A2, SLC24A3, SLC24A4, SLC24A5, SLC24A6 |
| 25 | mitochondrial carriers | SLC25A1, SLC25A2, SLC25A3, SLC25A4, SLC25A5, SLC25A6, SLC25A7, SLC25A8, SLC25A9, SLC25A10, SLC25A11, SLC25A12, SLC25A13, SLC25A14, SLC25A15, SLC25A16, SLC25A17, SLC25A18, SLC25A19, SLC25A20, SLC25A21, SLC25A22, SLC25A27 |
| 26 | multifunctional anion exchangers |
SLC26A1, SLC26A2, SLC26A3, SLC26A4, SLC26A5, SLC26A6, SLC26A7, SLC26A8, SLC26A9, SLC26A11 SLC26A10 is a pseudogene |
| 27 | fatty acid transporters (FATPs) | SLC27A1, SLC27A2, SLC27A3, SLC27A4, SLC27A5, SLC27A6 |
| 28 | Na+–dependent concentrative nucleoside transport (CNTs) | SLC28A1, SLC28A2, SLC28A3 |
| 29 | equilibrative nucleoside transporters (ENTs) | SLC29A1, SLC29A2, SLC29A3, SLC29A4 |
| 30 | efflux and compartmentalization of zinc (ZNTs) |
SLC30A1, SLC30A2, SLC30A3, SLC30A4, SLC30A5, SLC30A6, SLC30A7, SLC30A8, SLC30A9, SLC30A10
polymorphisms in the gene encoding SLC30A8 are associated with increased diabetes risk |
| 31 | copper transporters (CTRs) |
SLC31A1, SLC31A2 these mediate copper uptake ATP7A and ATP7B are related copper transporting ATPases that mediate copper export ATP7A is defective in Menkes disease and ATP7B is defective in Wilson disease |
| 32 | vesicular inhibitory amino acid transporter (VIAAT) |
SLC32A1 also called vesicular GABA transporter (VGAT) |
| 33 | acetyl-CoA transporter (ACATN) | SLC33A1 |
| 34 | type II Na+–phosphate cotransporters | SLC34A1, SLC34A2, SLC34A3 |
| 35 | nucleoside sugar transporters |
at least 17 family members in humans divided into
five subfamilies identified as A through E
SLC35C1 is also identified as the GDP-fucose transporter (gene symbol = FUCT) |
| 36 | proton–coupled amino acid transporters | SLC36A1, SLC36A2, SLC36A3, SLC36A4 |
| 37 | sugar–phosphate/phosphate exchangers (SPXs) |
SLC37A1, SLC37A2, SLC37A3, SLC37A4 SLC37A4 is also known as glucose-6-phosphate transporter-1 (G6PT1) which is defective in glycogen storage disease type1b |
| 38 | sodium–coupled neutral amino acid (system N/A) transporters (SNATs) |
System A family includes
SLC38A1, SLC38A2, SLC38A4 System N family includes SLC38A3, SLC38A5, SLC38A6 |
| 39 | metal ion transporters (ZIPs) | SLC39A1, SLC39A2, SLC39A3, SLC39A4, SLC39A5, SLC39A6, SLC39A7, SLC39A8, SLC39A9, SLC39A10, SLC39A11, SLC39A12, SLC39A13, SLC39A14 |
| 40 | basolateral iron transporter | SLC40A1is more commonly known as as ferroportin, but is also known as iron-regulated gene 1 (IREG1), reticuloendothelial iron transporter (MTP1); was also identified as SLC11A3 which is no longer used |
| 41 | MgtE–like magnesium transporters |
SLC41A1, SLC41A2, SLC41A3
MgtE is a divalent cation transporter first identified in the bacteria Chlamydomonas reinhardtii |
| 42 | Rh ammonium transporters |
SLC42A1, SLC42A2, SLC42A3 also identified as RhAG, RhBG, RhCG these transporters are named for the Rh blood–group antigens; e.g. RhAG is encoded by the RHAG gene which is also designated as the CD241 gene (cluster of differentiation 241) |
| 43 | Na+–independent, system–L like amino acid transporters | SLC43A1, SLC43A2, SLC43A3 |
| 44 | chlorine–like transporters | SLC44A1, SLC44A2, SLC44A3, SLC44A4, SLC44A5 |
| 45 | putative sugar transporters | SLC45A1, SLC45A2, SLC54A3, SLC45A4 |
| 46 | heme transporters | SLC46A1, SLC46A2 |
| 47 | multidrug and toxin extrusion | SLC47A1, SLC47A2 |
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Clinical Significances of Transporter Defects
As might be expected, defects in the expression and/or function of membrane transporters leads to the manifestation of numerous clinical disorders. It is not the intention of this section to cover all disorders related to defects in membrane transporters but to highlight several with emphasis on diseases that have been mentioned throughout the pages of this web site or in specific disease discussion pages.
ABCA1 is involved in the transport of cholesterol out of cells when HDLs are bound to their cell surface receptor, SR-B1 (see the Lipoproteins page for more details). One important consequence of the activity of ABCA1 in macrophages is that the efflux of cholesterol results in a suppression of inflammatory responses triggered by macrophages that have become foam cells due to cholesterol uptake. Defects in ABCA1 result in Tangier disease which is characterized by two clinical hallmarks; enlarged lipid-laden tonsils and low serum HDL.
ABCB4 is a member of the P-glycoprotein family of multidrug resistance transporters. Defects in the ABCB4 gene are associated with 6 liver diseases: progressive familial intrahepatic cholestasis type 3 (PFIC3), adult biliary cirrhosis, transient neonatal cholestasis, drug-induced cholestasis, intrahepatic cholestasis of pregnancy, and low phospholipid-associated cholelithiasis syndrome.
ABCB7 is a protein localized to the inner mitochondrial membrane and is involved in iron homeostasis. Defects in the gene are associated with X-linked sideroblastic anemia with ataxia (XSAT) which is characterized by an infantile to early childhood onset of non-progressive cerebellar ataxia and mild anemia with hypochromia and microcytosis.
ABCB11 is also known as bile salt export protein (BSEP) which is involved in bile salt transport out of hepatocytes. Defects in the gene are associated with progressive familial intrahepatic cholestasis type 2 (PFIC2).
ABCC2 was first identified as the canalicular multispecific organic anion transporter (CMOAT) and is also called multidrug resistance associated protein 2 (MRP2). Defects in the gene encoding ABCC2 result in Dubin-Johnson syndrome which is a form of conjugated hyperbilirubinemia.
ABCD1 is involved in the import and/or anchoring of very long-chain fatty acid-CoA synthetase (VLCFA-CoA synthetase) to the peroxisomes. Defects in the result in X-linked adrenoleukodystrophy (XALD).
ATP7A and ATP7B are copper transporting ATPases that are related to SLC31A1. Defects in ATP7A result in Menkes disease and defects in ATP7B are associated with Wilson disease.
Defects in many members of the SLC6 family are associated with mental retardation, affective disorders, and other neurological dysfunctions. SLC6A1 defects are associated with epilepsy and schizophrenia. SLC6A2 defects are associated with depression and anorexia nervosa. SLC6A3 defects are associated with Parkinsonism, Tourette syndrome, ADHD, and substance abuse. SLC6A4 defects are associated with anxiety disorder, depression , autism, and substance abuse.
SLC6A19 is also called system B(0) neutral amino acid transporter 1 [B(0)AT1]. This transporter is involved in neutral amino acid transport with highest levels of expression in the kidney and small intestine. Deficiency in SLC6A19 leads to Hartnup disease which results from impaired transport of neutral amino acids across epithelial cells in renal proximal tubules and intestinal mucosa. Symptoms include transient manifestations of pellagra-like light sensitive rash, cerebellar ataxia, and psychosis.
SLC11A2, which is also known as DMT1 (divalent metal-ion transporter-1), is involved in the uptake of iron by the apical surface of the duodenum. In addition to iron, DMT1 is involved in manganese, cobalt, cadmium, nickel, copper, and zinc transport. Defects in DMT1 activity are associated with hypochromic microcytic anemia with iron overload.
SLC12A6 defects are associated with Andermann syndrome which is also referred to as agenesis of corpus callosum and peripheral neuropathy (ACCPN). The disorder gets its name from the fact that afflicted individuals have sensory and motor neuropathy associated with agenesis of the corpus callosum. The incidence of this disorder is high French Canadians living in the Charlevoix region of Quebec.
SLC16A2 is defective in Allan-Herndon-Dudley syndrome (AHDS) which is characterized by marked by hypotonia, weakness, excessive drooling, reduced muscle mass, and delay of developmental milestones in infancy and early childhood. Afflicted individuals have severe impairment of cognitive development but do not exhibit any major malformations.
SLC22A18 is found in the imprinted region of chromosome 11 associated with Beckwith-Wiedemann syndrome (BWS)
SLC30A8 is involved in efflux of zinc and polymorphisms in the gene encoding this transporter are associated with increased risk of the development of type 2 diabetes. The functional defect in this protein result in impaired pancreatic β-cell function leading to defects in insulin secretion.
SLC35C1 is also known as the GDP-fucose transporter (gene symbol = FUCT1). Defects in the FUCT1 gene result in a congenital disorder of glycosylation (CDG). Specifically the disorder is a type II CDG identified as CDGIIc. Type II CDGs result from defects in the processing of N-linked glycoproteins. CDGIIc is also called leukocyte adhesion deficiency syndrome II (LAD II). LAD II is a primary immunodeficiency syndrome which manifests due to leukocyte dysfunction. Symptoms of LAD II include unique facial features, recurrent infections, persistent leukocytosis, defective neutrophil chemotaxis and severe growth and mental retardation.
SLC40A1 is a multiple transmembrane spanning protein involved in iron transport. The protein is highyl expressed in the intestine, liver, and reticuloendothelial cells. SLC40A1 is more commonly known as ferroportin or insulin-regulated gene 1 (IREG1). The protein is required for the transport of dietary iron across the basolateral membranes of intestinal enterocytes. Defects in the SLC40A1 gene are associated with type 4 hemochromatosis.
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Michael W. King, Ph.D / IU School of Medicine / miking at iupui.edu
