ATP-binding cassette transporters (ABC) are multipass transmembrane proteins that use the energy of ATP hydrolysis to transport substrates across membrane bilayers. Members of ABC transporter subfamily A are full-length transporters [], which consist of a single long polypeptide chain organised into two tandemly arranged halves. Each half contains a membrane-spanning domain (MSD) followed by a cytoplasmic nucleotide binding domain (NBD) []. Several members of this group have been shown to mediate the transport of a variety of physiologic lipid compounds, such as sterols, phospholipids and bile acids [, ].ABCA7 plays a role in clearance of apoptotic cells by affecting their phagocytosis []. In the human visual cycle, ABCA4 acts as an inward-directed retinoid flipase, retinoid substrates imported by ABCA4 from the extracellular or intradiscal (rod) membrane surfaces to the cytoplasmic membrane surface are all-trans-retinaldehyde (ATR) and N-retinyl-phosphatidyl-ethanolamine (NR-PE). Once transported to the cytoplasmic surface, ATR is reduced to vitamin A by trans-retinol dehydrogenase (tRDH) and then transferred to the retinal pigment epithelium (RPE) where it is converted to 11-cis-retinal. ABCA4 may also play a role in photoresponse, removing ATR/NR-PE from the extracellular photoreceptor surfaces during bleach recovery []. It has been suggested that ABCA9 plays a role in monocyte differentiation and lipid homeostasis [].
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 [].Retinal-specific ATP-binding cassette transporter ABCA4 (also known as the Rim protein, ABCR) is a eukaryotic protein belonging to the ABC-A subfamily of the ABC transporter family. In humans, ABCA4 is localised with opsin photopigments in outer segment disc membranes of rod and cone photoreceptor cells. It serves as an N-retinylidene-phosphatidylethanolamine and phosphatidylethanolamine importer []. Mutations in the ABCA4 gene cause Stargardt macular degeneration, a recessive disease characterised by the loss in central vision, progressive bilateral atrophy of photoreceptor and retinal pigment epithelial (RPE) cells, accumulation of fluorescent deposits in the macula, and a delay in dark adaptation [, ]. ABCR contains eight glycosylation sites. Four sites reside in a 600-amino acid exocytoplasmic domain of the N-terminal half between the first transmembrane segment H1 and the first multi-spanning membrane domain, and four sites are in a 275-amino acid domain of the C-terminal half between transmembrane segment H7 and the second multi-spanning membrane domain. This leads to a model in which each half has a transmembrane segment followed by a large exocytoplasmic domain, a multi-spanning membrane domain, and a nucleotide binding domain.
