Editing ABC transporter (section)

=== Mammalian subfamilies ===
There are 49 known ABC transporters present in humans, which are classified into seven families by the Human Genome Organization.

{| class="wikitable"
|-
! Family !! Members !! Function !! Examples
 |-
 | ABCA  || This family contains some of the largest transporters (over 2,100 amino acids long). Five of them are located in a cluster in the 17q24 chromosome. || Responsible for the transportation of cholesterol and lipids, among other things. || [[ABCA12]] [[ABCA1]]
 |-
 | ABCB || Consists of 4 full and 7 half transporters. || Some are located in the blood–brain barrier, liver, mitochondria, transports peptides and bile, for example. || [[ABCB5]]
 |-
 | ABCC || Consists of 12 full transporters.  || Used in ion transport, cell-surface receptors, toxin secretion.   Includes the CFTR protein, which causes [[cystic fibrosis]] when deficient. || [[ABCC6]]
 |-
 | ABCD || Consists of 4 half transporters || Are all used in [[peroxisome]]s. || [[ABCD1]]
 |-
 | ABCE/ABCF || Consists of 1 ABCE and 3 ABCF proteins. || These are not actually transporters but merely ATP-binding domains that were derived from the ABC family, but without the transmembrane domains. These proteins mainly regulate protein synthesis or expression.  || [[ABCE1]], [[ABCF1]], [[ABCF2]]
|-
 |-
 | ABCG || Consists of 6 "reverse" half-transporters, with the NBF at the NH<sub>3</sub><sup>+</sup> end and the TM at the COO- end. || Transports lipids, diverse drug substrates, bile, cholesterol, and other steroids. || [[ABCG2]] [[ABCG1]]
|}

A full list of human ABC transporters can be found from.<ref>{{cite journal |last1=Vasiliou |first1=V |last2=Vasiliou |first2=K |last3=Nebert |first3=DW |title=Human ATP-binding cassette (ABC) transporter family. |journal=Human Genomics |date=April 2009 |volume=3 |issue=3 |pages=281–90 |pmid=19403462 |pmc=2752038 |doi=10.1186/1479-7364-3-3-281 |doi-access=free }}</ref>

==== ABCA ====
The ABCA subfamily is composed of 12 full transporters split into two subgroups.  The first subgroup consists of seven genes that map to six different [[chromosome]]s.  These are [[ABCA1]], [[ABCA2]], [[ABCA3]], and [[ABCA4]], [[ABCA7]], [[ABCA12]], and [[ABCA13]]. The other subgroup consists of [[ABCA5]] and [[ABCA6]] and [[ABCA8]], [[ABCA9]] and [[ABCA10]]. A8-10.
All of subgroup 2 is organized into a head to tail cluster of chromosomes on [[chromosome 17 (human)|chromosome 17]]q24.  Genes in this second subgroup are distinguished from ABCA1-like genes by having 37-38 exons as opposed to the 50 exons in ABCA1.  
The ABCA1 subgroup is implicated in the development of genetic diseases.  In the recessive Tangier's disease, the [[ABCA1]] protein is mutated.  Also, the [[ABCA4]] maps to a region of chromosome 1p21 that contains the gene for Stargardt's disease.  This gene is found to be highly expressed in rod photoreceptors and is mutated in Stargardt's disease, recessive retinitis pigmentism, and the majority of recessive cone-rod dystrophy.<ref name="Dean_2001"/>

==== ABCB ====
The ABCB subfamily is composed of four full transporters and two half transporters.  This is the only human subfamily to have both half and full types of transporters.  [[ABCB1]] was discovered as a protein overexpressed in certain drug resistant tumor cells.  It is expressed primarily in the [[blood–brain barrier]] and liver and is thought to be involved in protecting cells from toxins.  Cells that overexpress this protein exhibit [[multi-drug resistance]].<ref name="Dean_2001"/>

==== ABCC ====
Subfamily ABCC contains thirteen members and nine of these transporters are referred to as the Multidrug Resistance Proteins (MRPs). The MRP proteins are found throughout nature and they mediate many important functions.<ref name="Chen_2011">{{cite journal | vauthors = Chen ZS, Tiwari AK | title = Multidrug resistance proteins (MRPs/ABCCs) in cancer chemotherapy and genetic diseases | journal = The FEBS Journal | volume = 278 | issue = 18 | pages = 3226–45 | date = Sep 2011 | pmid = 21740521 | pmc = 3168698 | doi = 10.1111/j.1742-4658.2011.08235.x }}</ref> They are known to be involved in ion transport, toxin secretion, and signal transduction.<ref name="Dean_2001"/> Of the nine MRP proteins, four of them, MRP4, 5, 8, 9, (ABCC4, 5, 11, and 12),  have a typical ABC structure with four domains, comprising two membrane spanning domains, with each spanning domain followed by a nucleotide binding domain.   These are referred to as short MRPs. The remaining 5 MRP's (MRP1, 2, 6, 7) (ABCC1, 2, 3, 6 and 10) are known as long MRPs and feature an additional fifth domain at their [[N terminus]].<ref name="Chen_2011"/>

[[Cystic fibrosis transmembrane conductance regulator|CFTR]], the transporter involved in the disease [[cystic fibrosis]], is also considered part of this subfamily. Cystic fibrosis occurs upon mutation and loss of function of CFTR.<ref name="Dean_2001"/>

The [[sulfonylurea receptor|sulfonylurea receptors (SUR)]], involved in insulin secretion, neuronal function, and muscle function, are also part of this family of proteins. Mutations in SUR proteins are a potential cause of [[Neonatal diabetes mellitus]]. SUR is also the binding site for drugs such as [[sulfonylureas]] and potassium-channel openers activators such as [[diazoxide]].

==== ABCD ====
The ABCD subfamily consists of four genes that encode half transporters expressed exclusively in the [[peroxisome]].  [[ABCD1]] is responsible for the X-linked form of [[Adrenoleukodystrophy]] (ALD) which is a disease characterized by neurodegeneration and adrenal deficiency that typically is initiated in late childhood.  The cells of ALD patients feature accumulation of unbranched saturated fatty acids, but the exact role of [[ABCD1]] in the process is still undetermined.  In addition, the function of other ABCD genes have yet to be determined but have been thought to exert related functions in [[fatty acid metabolism]].<ref name="Dean_2001"/>

==== ABCE and ABCF ====
Both of these subgroups are composed of genes that have ATP binding domains that are closely related to other ABC transporters, but these genes do not encode for trans-membrane domains.  ABCE consists of only one member, OABP or [[ABCE1]], which is known to recognize certain [[oligodendrocytes]] produced in response to certain viral infections.  Each member of the ABCF subgroup consist of a pair of ATP binding domains.<ref name="Dean_2001"/>

==== ABCG ====
Six half transporters with ATP binding sites on the N terminus and trans-membrane domains at the C terminus make up the ABCG subfamily.  This orientation is opposite of all other ABC genes. There are only 5 ABCG genes in the human genome, but there are 15 in the Drosophila genome and 10 in yeast.  
The ABCG2 gene was discovered in cell lines selected for high level resistance for [[mitoxantrone]] and no expression of [[ABCB1]] or [[ABCC1]]. [[ABCG2]] can export [[anthracycline]] anticancer drugs, as well as [[topotecan]], [[mitoxantrone]], or [[doxorubicin]] as substrates. [[Chromosomal translocations]] have been found to cause the ABCG2 amplification or rearrangement found in resistant cell lines.<ref name="Dean_2001"/>