Table 1. Human ATP binding cassette... | American Society of Hematology

Table 1.

Human ATP binding cassette transporters

Name	Functional description
ABC1: subfamily A
ABC1	ABCA1 is a major regulator of cellular cholesterol and phospholipid homeostasis. It mediates, for example, the efflux of phospholipids (PS) and cholesterol from macrophages to apoA-1, reversing foam cell formation. Likely not involved in hepatic cholesterol secretion and intestinal apical cholesterol transport.
ABCA2	ABCA2 is a regulator of steroid and lipid transport, primarily in neural membranes.
ABCA3	ABCA3 is expressed almost exclusively in the lung tissue and is most likely involved in the formation of surfactant protein (SP) in lamellar bodies in the lungs.
ABCR	This protein is a retina-specific ABC transporter with N-retinylidene-PE (phosphatidylethanolamine) as a substrate. It is expressed exclusively in retina photoreceptor cells, indicating the gene product mediates transport of an essential molecule across the photoreceptor cell membrane.
ABCA5	No information available about this protein.
ABCA6	Actual function and substrate are unknown, though it is speculated, based on certain structural features and responses in regulation to cholesterol, that it is involved in the homeostasis of macrophage lipids.
ABCA7	This full transporter has been detected predominantly in myelo-lymphatic tissues with the highest expression in peripheral leukocytes, thymus, spleen, and bone marrow. The function of this protein is not yet known; however, the expression pattern suggests a role in lipid homeostasis in cells of the immune system. Alternative splicing of this gene results in 2 transcript variants.
ABCA8	This gene is clustered among 4 other ABC1 family members on 17q24, but neither the substrate nor the function of this gene is known.
ABCA9	This gene is clustered among 4 other ABC1 family members on 17q24 and may play a role in monocyte differentiation and macrophage lipid homeostasis.
ABCA10	This gene is clustered among 4 other ABC1 family members on 17q24, but neither the substrate nor the function of this gene is known. ABCA10 expression is suppressed by cholesterol import into macrophages, indicating that it is a cholesterol-responsive gene.
ABCA12	No information available about this protein.
ABCA13	The predicted ABCA13 protein consists of 5058 amino acid residues, making it the largest ABC protein described to date. ABCA13 contains a hydrophobic, predicted transmembrane segment at the N-terminus, followed by a large hydrophilic region.
MDR/TAP: subfamily B
ABCB1	The protein (also called P-glycoprotein) is an ATP-dependent drug efflux pump for xenobiotic compounds with broad substrate specificity. It is responsible for decreased drug accumulation in multidrug-resistant cells and often mediates the development of resistance to anticancer drugs.
ABCB2	The protein is a half-ABC transporter functioning as a peptide transporter involved in the pumping of degraded cytosolic peptides across the endoplasmic reticulum into the membrane-bound compartment where class I molecules assemble.
ABCB3	The protein is a half-abc transporter functioning as a peptide transporter involved in antigen presentation. It forms a heterodimer with TAP1/ABCB2 in order to transport peptides from the cytoplasm to the endoplasmic reticulum. Alternative splicing of this gene produces 2 products which differ in peptide selectivity and level of restoration of surface expression of MHC class I molecules.
ABCB4	Most likely involved in biliary phosphatidylcholine secretion from hepatocytes in a bile salt-dependent manner. Biliary phosphatidylcholine (PC) is necessary to keep the bile “nontoxic”.
ABCB5	No information available about this protein.
ABCB6	This half-transporter likely plays a role in mitochondrial function and possibly transports iron.
ABCB7	This gene encodes a half-transporter involved in the transport of heme from the mitochondria to the cytosol. With iron/sulfur cluster precursors as its substrates, this protein may play a role in metal homeostasis.
ABCB8	The function of this half-transporter has not yet been determined; however, it may involve the compartmentalization and transport of heme, as well as peptides, from the mitochondria to the nucleus and cytosol. This protein may also play a role in the transport of phospholipids into mitochondrial membranes.
ABCB9	The function of this half-transporter has not yet been determined; however, this protein may play a role in lysosomes. Alternative splicing of this gene results in 2 known products, which are likely to have different substrate specifications.
ABCB10	No information available about this protein.
BSEP	BSEP is the major canalicular bile salt export pump in humans, and is responsible for active transport of bile salts across the hepatocyte canalicular membrane into bile. It represents the molecular basis of the bile salt—dependent bileflow. BSEP activity is necessary for PC secretion via PGY3/ABCB4.
CFTR/MPR: subfamily C
MRP1	MRP1 functions as a multispecific organic anion transporter, with (oxidized) glutathione, cysteinyl leukotrienes, and activated aflatoxin B1 as substrates. This protein also transports glucuronides and sulfate conjugates of steroid hormones and bile salts. It also transports drugs and other hydrophobic compounds in the presence of glutathione.
MRP2	MRP2 is expressed in the canalicular (apical) part of the hepatocyte and functions in biliary transport of mainly anionic conjugates with glutathione, sulfate, or glucuronosyl (eg, glucuronosyl bilirubin). Other substrates include anticancer drugs such as vinblastine (similar specificity as MRP1/ABCC1); appears to contribute to drug resistance.
MRP3	The specific function of this protein has not yet been determined; however, this protein may play a role in the transport of biliary and intestinal excretion of organic anions including bile salts.
MRP4	The human multidrug resistance protein MRP4 is an organic anion transporter that transports cyclic nucleotides and some nucleoside monophosphate analogs including nucleoside-based antiviral drugs (specificity similar to MRP5). MRP4 also transports prostaglandins.
MRP5	The human multidrug resistance protein MRP5 is an organic anion transporter that transports cyclic nucleotides and some nucleoside monophosphate analogs including nucleoside-based antiviral drugs (specificity similar to MRP4).
MRP6	In humans, MRP6 is highly expressed in the liver and kidney. Lower expression was found in tissues affected by pseudoxanthoma elasticum, including skin, retina, and vessel walls. Functional studies suggest that small peptides (BQ123) are transported by rat Mrp6. Recent studies show transport of glutathione conjugates.
CFTR/MPR: subfamily C
CFTR	This protein functions as a chloride channel and controls the regulation of other transport pathways.
SUR1	This protein functions as a modulator of ATP-sensitive potassium channels and insulin release.
SUR2	This protein is thought to form ATP-sensitive potassium channels in cardiac, skeletal, and vascular and nonvascular smooth muscle. Protein structure suggests a role as the drug-binding channel-modulating subunit of the extrapancreatic ATP-sensitive potassium channels.
MRP7	MRP7/ABCC7 has been shown to transport estradiol(2)17beta glucuronide and also LTC4, although not as well. E(2)17betaG transport was saturable, with Km and Vmax values of 57.8 μM and 53.1 pmol/mg/min, respectively.
ABCC11	MRP8/ABCC11 is expressed at low levels in all tissues, except kidney, spleen, and colon. This gene and family member ABCC12 are determined to be derived by duplication and are both localized to chromosome 16q12.1. Their chromosomal localization, potential function, and expression patterns identify them as candidates for paroxysmal kinesigenic choreoathetosis, a disorder characterized by attacks of involuntary movements and postures, chorea, and dystonia. Multiple alternatively spliced transcript variants have been described for this gene.
ABCC12	ABCC12 is expressed at low levels in testes, ovary, and prostate tissues. This gene and family member ABCC11 are determined to be derived by duplication and are both localized to chromosome 16q12.1. Multiple alternatively spliced transcript variants encoding different isoforms have been described for this gene but some of their full-length sequences are not available.
ABCC13	ABCC13 is highly expressed in human fetal liver cells. Levels in cells that were induced to differentiate by TPA decreased markedly. This evidence suggests that ABCC13 may be involved in hematopoiesis.
ALD: subfamily D
ALD	This protein is a member of the ALD subfamily, which is involved in peroxisomal import of fatty acids and/or fatty acyl-CoAs (coenzyme As) in the organelle. All known peroxisomal ABC transporters are half-transporters, which require a partner half-transporter molecule to form a functional homodimeric or heterodimeric transporter. This peroxisomal membrane protein is likely involved in the peroxisomal transport or catabolism of very long chain fatty acids.
ALDL1	This protein is a member of the ALD subfamily, which is involved in peroxisomal import of fatty acids and/or fatty acyl-CoAs in the organelle. All known peroxisomal ABC transporters are half-transporters, which require a partner half-transporter molecule to form a functional homodimeric or heterodimeric transporter. The function of this peroxisomal membrane protein is unknown; however, this protein is speculated to function as a dimerization partner of ABCD1 and/or other peroxisomal ABC transporters.
PXMP1	This protein is a member of the ALD subfamily, which is involved in peroxisomal import of fatty acids and/or fatty acyl-CoAs in the organelle. All known peroxisomal ABC transporters are half-transporters, which require a partner half-transporter molecule to form a functional homodimeric or heterodimeric transporter.
PXMPIL	This protein is a member of the ALD subfamily, which is involved in peroxisomal import of fatty acids and/or fatty acyl-CoAs in the organelle. The function of this peroxisomal membrane protein is unknown. However, it is speculated that it may function as a heterodimer for another peroxisomal ABC transporter and, therefore, may modify the adrenoleukodystrophy phenotype. It may also play a role in the process of peroxisome biogenesis.
OABP: subfamily E
RNASEL1	This protein is a member of the OABP subfamily. Alternatively referred to as the RNase L inhibitor, this protein functions to block the activity of ribonuclease L. Activation of ribonuclease L leads to inhibition of protein synthesis in the 2-5A/RNase L system, the central pathway for viral interferon action.
GNC20: subfamily F
ABC50	Unlike other members of the superfamily, this protein lacks the transmembrane domains that are characteristic of most ABC transporters. This protein may be regulated by tumor necrosis factor-α and play a role in enhancement of protein synthesis and the inflammation process.
ABCF2	No information available about this protein.
ABCF3	No information available about this protein.
White: subfamily G
ABCG1	ABCG1 is involved in macrophage cholesterol efflux and may regulate cellular lipid homeostasis in other cell types.
ABCG2	This protein functions as a xenobiotic transporter, which may play a major role in multidrug resistance. It likely serves as a cellular defense mechanism in response to mitoxantrone and anthracycline exposure. Recently it has been shown to transport organic anions but also steroids (cholesterol, estradiol, progesterone, testosterone) and certain chlorophyll metabolites.
ABCG4	Given ABCG4's sequential similarity to ABCG1 and its similarity to other members of the ABCG family, ABCG4 is most likely involved in cholesterol transport in the brain.
ABCG5	ABCG5 functions as a half-transporter to limit intestinal absorption and promote biliary excretion of sterols. It is expressed in a tissue-specific manner in the liver, colon, and intestine. This gene is tandemly arranged on chromosome 2, in a head-to-head orientation with family member ABCG8.
ABCG8	ABCG8 functions as a half-transporter to limit intestinal absorption and promote biliary excretion of sterols. It is expressed in a tissue-specific manner in the liver, colon, and intestine. This gene is tandemly arranged on chromosome 2, in a head-to-head orientation with family member ABCG5.
ANSA: subfamily H
ANSAI	Human arsenite transporter ANSAI is a dimeric ABC ATPase with a nucleotide-binding domain and a transmembrane domain. Although its function in humans is not very well known, it is known that this protein functions as an arsenite and antimonite exporter pump in E coli.
ANSAII	The function of ANSAII is not completely understood, but its 98% identity with ANSAI, and homology with ABC proteins in other species whose functions are well understood, suggests that its function is the same as that of ANSAI.

Name	Functional description
ABC1: subfamily A
ABC1	ABCA1 is a major regulator of cellular cholesterol and phospholipid homeostasis. It mediates, for example, the efflux of phospholipids (PS) and cholesterol from macrophages to apoA-1, reversing foam cell formation. Likely not involved in hepatic cholesterol secretion and intestinal apical cholesterol transport.
ABCA2	ABCA2 is a regulator of steroid and lipid transport, primarily in neural membranes.
ABCA3	ABCA3 is expressed almost exclusively in the lung tissue and is most likely involved in the formation of surfactant protein (SP) in lamellar bodies in the lungs.
ABCR	This protein is a retina-specific ABC transporter with N-retinylidene-PE (phosphatidylethanolamine) as a substrate. It is expressed exclusively in retina photoreceptor cells, indicating the gene product mediates transport of an essential molecule across the photoreceptor cell membrane.
ABCA5	No information available about this protein.
ABCA6	Actual function and substrate are unknown, though it is speculated, based on certain structural features and responses in regulation to cholesterol, that it is involved in the homeostasis of macrophage lipids.
ABCA7	This full transporter has been detected predominantly in myelo-lymphatic tissues with the highest expression in peripheral leukocytes, thymus, spleen, and bone marrow. The function of this protein is not yet known; however, the expression pattern suggests a role in lipid homeostasis in cells of the immune system. Alternative splicing of this gene results in 2 transcript variants.
ABCA8	This gene is clustered among 4 other ABC1 family members on 17q24, but neither the substrate nor the function of this gene is known.
ABCA9	This gene is clustered among 4 other ABC1 family members on 17q24 and may play a role in monocyte differentiation and macrophage lipid homeostasis.
ABCA10	This gene is clustered among 4 other ABC1 family members on 17q24, but neither the substrate nor the function of this gene is known. ABCA10 expression is suppressed by cholesterol import into macrophages, indicating that it is a cholesterol-responsive gene.
ABCA12	No information available about this protein.
ABCA13	The predicted ABCA13 protein consists of 5058 amino acid residues, making it the largest ABC protein described to date. ABCA13 contains a hydrophobic, predicted transmembrane segment at the N-terminus, followed by a large hydrophilic region.
MDR/TAP: subfamily B
ABCB1	The protein (also called P-glycoprotein) is an ATP-dependent drug efflux pump for xenobiotic compounds with broad substrate specificity. It is responsible for decreased drug accumulation in multidrug-resistant cells and often mediates the development of resistance to anticancer drugs.
ABCB2	The protein is a half-ABC transporter functioning as a peptide transporter involved in the pumping of degraded cytosolic peptides across the endoplasmic reticulum into the membrane-bound compartment where class I molecules assemble.
ABCB3	The protein is a half-abc transporter functioning as a peptide transporter involved in antigen presentation. It forms a heterodimer with TAP1/ABCB2 in order to transport peptides from the cytoplasm to the endoplasmic reticulum. Alternative splicing of this gene produces 2 products which differ in peptide selectivity and level of restoration of surface expression of MHC class I molecules.
ABCB4	Most likely involved in biliary phosphatidylcholine secretion from hepatocytes in a bile salt-dependent manner. Biliary phosphatidylcholine (PC) is necessary to keep the bile “nontoxic”.
ABCB5	No information available about this protein.
ABCB6	This half-transporter likely plays a role in mitochondrial function and possibly transports iron.
ABCB7	This gene encodes a half-transporter involved in the transport of heme from the mitochondria to the cytosol. With iron/sulfur cluster precursors as its substrates, this protein may play a role in metal homeostasis.
ABCB8	The function of this half-transporter has not yet been determined; however, it may involve the compartmentalization and transport of heme, as well as peptides, from the mitochondria to the nucleus and cytosol. This protein may also play a role in the transport of phospholipids into mitochondrial membranes.
ABCB9	The function of this half-transporter has not yet been determined; however, this protein may play a role in lysosomes. Alternative splicing of this gene results in 2 known products, which are likely to have different substrate specifications.
ABCB10	No information available about this protein.
BSEP	BSEP is the major canalicular bile salt export pump in humans, and is responsible for active transport of bile salts across the hepatocyte canalicular membrane into bile. It represents the molecular basis of the bile salt—dependent bileflow. BSEP activity is necessary for PC secretion via PGY3/ABCB4.
CFTR/MPR: subfamily C
MRP1	MRP1 functions as a multispecific organic anion transporter, with (oxidized) glutathione, cysteinyl leukotrienes, and activated aflatoxin B1 as substrates. This protein also transports glucuronides and sulfate conjugates of steroid hormones and bile salts. It also transports drugs and other hydrophobic compounds in the presence of glutathione.
MRP2	MRP2 is expressed in the canalicular (apical) part of the hepatocyte and functions in biliary transport of mainly anionic conjugates with glutathione, sulfate, or glucuronosyl (eg, glucuronosyl bilirubin). Other substrates include anticancer drugs such as vinblastine (similar specificity as MRP1/ABCC1); appears to contribute to drug resistance.
MRP3	The specific function of this protein has not yet been determined; however, this protein may play a role in the transport of biliary and intestinal excretion of organic anions including bile salts.
MRP4	The human multidrug resistance protein MRP4 is an organic anion transporter that transports cyclic nucleotides and some nucleoside monophosphate analogs including nucleoside-based antiviral drugs (specificity similar to MRP5). MRP4 also transports prostaglandins.
MRP5	The human multidrug resistance protein MRP5 is an organic anion transporter that transports cyclic nucleotides and some nucleoside monophosphate analogs including nucleoside-based antiviral drugs (specificity similar to MRP4).
MRP6	In humans, MRP6 is highly expressed in the liver and kidney. Lower expression was found in tissues affected by pseudoxanthoma elasticum, including skin, retina, and vessel walls. Functional studies suggest that small peptides (BQ123) are transported by rat Mrp6. Recent studies show transport of glutathione conjugates.
CFTR/MPR: subfamily C
CFTR	This protein functions as a chloride channel and controls the regulation of other transport pathways.
SUR1	This protein functions as a modulator of ATP-sensitive potassium channels and insulin release.
SUR2	This protein is thought to form ATP-sensitive potassium channels in cardiac, skeletal, and vascular and nonvascular smooth muscle. Protein structure suggests a role as the drug-binding channel-modulating subunit of the extrapancreatic ATP-sensitive potassium channels.
MRP7	MRP7/ABCC7 has been shown to transport estradiol(2)17beta glucuronide and also LTC4, although not as well. E(2)17betaG transport was saturable, with Km and Vmax values of 57.8 μM and 53.1 pmol/mg/min, respectively.
ABCC11	MRP8/ABCC11 is expressed at low levels in all tissues, except kidney, spleen, and colon. This gene and family member ABCC12 are determined to be derived by duplication and are both localized to chromosome 16q12.1. Their chromosomal localization, potential function, and expression patterns identify them as candidates for paroxysmal kinesigenic choreoathetosis, a disorder characterized by attacks of involuntary movements and postures, chorea, and dystonia. Multiple alternatively spliced transcript variants have been described for this gene.
ABCC12	ABCC12 is expressed at low levels in testes, ovary, and prostate tissues. This gene and family member ABCC11 are determined to be derived by duplication and are both localized to chromosome 16q12.1. Multiple alternatively spliced transcript variants encoding different isoforms have been described for this gene but some of their full-length sequences are not available.
ABCC13	ABCC13 is highly expressed in human fetal liver cells. Levels in cells that were induced to differentiate by TPA decreased markedly. This evidence suggests that ABCC13 may be involved in hematopoiesis.
ALD: subfamily D
ALD	This protein is a member of the ALD subfamily, which is involved in peroxisomal import of fatty acids and/or fatty acyl-CoAs (coenzyme As) in the organelle. All known peroxisomal ABC transporters are half-transporters, which require a partner half-transporter molecule to form a functional homodimeric or heterodimeric transporter. This peroxisomal membrane protein is likely involved in the peroxisomal transport or catabolism of very long chain fatty acids.
ALDL1	This protein is a member of the ALD subfamily, which is involved in peroxisomal import of fatty acids and/or fatty acyl-CoAs in the organelle. All known peroxisomal ABC transporters are half-transporters, which require a partner half-transporter molecule to form a functional homodimeric or heterodimeric transporter. The function of this peroxisomal membrane protein is unknown; however, this protein is speculated to function as a dimerization partner of ABCD1 and/or other peroxisomal ABC transporters.
PXMP1	This protein is a member of the ALD subfamily, which is involved in peroxisomal import of fatty acids and/or fatty acyl-CoAs in the organelle. All known peroxisomal ABC transporters are half-transporters, which require a partner half-transporter molecule to form a functional homodimeric or heterodimeric transporter.
PXMPIL	This protein is a member of the ALD subfamily, which is involved in peroxisomal import of fatty acids and/or fatty acyl-CoAs in the organelle. The function of this peroxisomal membrane protein is unknown. However, it is speculated that it may function as a heterodimer for another peroxisomal ABC transporter and, therefore, may modify the adrenoleukodystrophy phenotype. It may also play a role in the process of peroxisome biogenesis.
OABP: subfamily E
RNASEL1	This protein is a member of the OABP subfamily. Alternatively referred to as the RNase L inhibitor, this protein functions to block the activity of ribonuclease L. Activation of ribonuclease L leads to inhibition of protein synthesis in the 2-5A/RNase L system, the central pathway for viral interferon action.
GNC20: subfamily F
ABC50	Unlike other members of the superfamily, this protein lacks the transmembrane domains that are characteristic of most ABC transporters. This protein may be regulated by tumor necrosis factor-α and play a role in enhancement of protein synthesis and the inflammation process.
ABCF2	No information available about this protein.
ABCF3	No information available about this protein.
White: subfamily G
ABCG1	ABCG1 is involved in macrophage cholesterol efflux and may regulate cellular lipid homeostasis in other cell types.
ABCG2	This protein functions as a xenobiotic transporter, which may play a major role in multidrug resistance. It likely serves as a cellular defense mechanism in response to mitoxantrone and anthracycline exposure. Recently it has been shown to transport organic anions but also steroids (cholesterol, estradiol, progesterone, testosterone) and certain chlorophyll metabolites.
ABCG4	Given ABCG4's sequential similarity to ABCG1 and its similarity to other members of the ABCG family, ABCG4 is most likely involved in cholesterol transport in the brain.
ABCG5	ABCG5 functions as a half-transporter to limit intestinal absorption and promote biliary excretion of sterols. It is expressed in a tissue-specific manner in the liver, colon, and intestine. This gene is tandemly arranged on chromosome 2, in a head-to-head orientation with family member ABCG8.
ABCG8	ABCG8 functions as a half-transporter to limit intestinal absorption and promote biliary excretion of sterols. It is expressed in a tissue-specific manner in the liver, colon, and intestine. This gene is tandemly arranged on chromosome 2, in a head-to-head orientation with family member ABCG5.
ANSA: subfamily H
ANSAI	Human arsenite transporter ANSAI is a dimeric ABC ATPase with a nucleotide-binding domain and a transmembrane domain. Although its function in humans is not very well known, it is known that this protein functions as an arsenite and antimonite exporter pump in E coli.
ANSAII	The function of ANSAII is not completely understood, but its 98% identity with ANSAI, and homology with ABC proteins in other species whose functions are well understood, suggests that its function is the same as that of ANSAI.

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