This entry represents ATP-binding cassette sub-family C member 4 (MRP4 or ABCC4) from animals. It belongs to the MRP (multidrug resistance protein) subfamily of the ATP-binding cassette (ABC) transporter family []. MRP4 is a broad specificity organic anion exporter that seems to be able to mediate the transport of conjugated steroids, prostaglandins, and glutathione []. It can localise to both basolateral and apical membranes in polarised cells, depending on the tissue where it is found []. Together with CFTR, MRP4 functions in the regulation of cAMP and beta-adrenergic contraction in cardiac myocytes []. MRP4 has been implicated in the high proliferative growth of some tumors including prostate tumors and neuroblastoma [, ]. It confers resistance to anticancer agents including thiopurine analogs, MTX and topotecan []. The ABC transporter family is a group of membrane proteins that use the hydrolysis of ATP to power the translocation of a wide variety of substrates across cellular membranes. ABC transporters minimally consist of two conserved regions: a highly conserved nucleotide-binding domain (NBD) and a less conserved transmembrane domain (TMD). Eukaryotic ABC proteins are usually organised either as full transporters (containing two NBDs and two TMDs), or as half transporters (containing one NBD and one TMD), that have to form homo- or heterodimers in order to constitute a functional protein [].
