Other ABC Transporter

The breast cancer resistance protein (BCRP) belongs to the G-branch of the ABC-transporter family (ABCG2). In contrast to most other ABC-proteins, BCRP consists of only one transmembrane domain (TDM) with one nucleotide binding fold (NBF) at its C-terminus. Because of this structural characteristic BCRP as well as other ABC-transporters with only one TMD are termed half transporters. To achieve functional activity these transporters have to form hetero- or homodimers. BCRP is involved in the multidrug resistance of certain tumors and transports endogenous compounds like cholesterol and steroid hormones. 

All ABC transporters have two nucleotide-binding domains (NBDs) and two transmembrane domains (Fig. 11-41). In some cases, all these domains are in a single long polypeptide other ABC transporters have two subunits, each contributing an NBD and a domain with six (or in some cases ten) transmembrane helices. Although many of the ABC transporters are in the plasma membrane, some types are also found in the endoplasmic reticulum and in the membranes of mitochondria and lysosomes. Most ABC transporters act as pumps, but at least some members of the superfamily act as ion channels that are opened and closed by ATP hydrolysis. The CFTR transporter (Box 11-3) is a Cl- channel operated by ATP hydrolysis. 

Further ABC Transporters. The induction of some other ABC transporters of the MRP subfamily has been reported in single studies. Mouse liver Mrpl is induced by TCPOBOP, depending on the expression of CAR [77], However, this was not confirmed by further studies using CAR activators including TCPOBOP in rats and mice [109,105]. Human hepatocytes treated with phenobarbital and rifampin also did not show an increased expression of MRP1, although in this study cultures... 

BCRP is a recently identified ABC transporter, originally identified by its ability to confer drug resistance that is independent of other ABC transporters including Pgp. Because it contains a single N-terminal ATP binding cassette, BCRP is referred to as a half-transporter. Nevertheless, BCRP is present in various tissues including the intestine, liver, kidney, brain, and placenta. Like... 

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