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ABCA4
ATP-binding cassette, sub-family A (ABC1), member 4, also known as ABCA4 or ABCR, is a protein which in humans is encoded by the ''ABCA4'' gene.〔(【引用サイトリンク】 url = http://www.ncbi.nlm.nih.gov/sites/entrez?Db=gene&Cmd=ShowDetailView&TermToSearch=24 )〕
ABCA4 is a member of the ATP-binding cassette transporter gene sub-family A (ABC1) found exclusively in multicellular eukaryotes.〔 The gene was first cloned and characterized in 1997 as a gene that causes Stargardt disease, an autosomal recessive disease that causes macular degeneration. The ''ABCA4'' gene transcribes a large retina-specific protein with two transmembrane domains (TMD), two glycosylated extracellular domains (ECD), and two nucleotide-binding domains (NBD). The ABCA4 protein is almost exclusively expressed in retina localizing in outer segment disk edges of rod photoreceptors.
== Structure ==
Previously known as the photoreceptor rim protein RmP or ABCR, the recently proposed ABCA4 structure consists of two transmembrane domains (TMDs), two large glycosylated extracellular domains (ECD), and two internal nucleotide binding domains (NBDs). One TMD spans across membranes with six units of protein linked together to form a domain. The TMDs are usually not conserved across genomes due to its specificity and diversity in function as channels or ligand-binding controllers. However, NBDs are highly conserved across different genomes—an observation consistent with which it binds and hydrolyzes ATP. NBD binds adenosine triphosphate molecules (ATP) to utilize the high-energy inorganic phosphate to carry out change in conformation of the ABC transporter. Transcribed ''ABCA4'' forms into a heterodimer: the two dimerized compartments of the channel are different from each other. When TMDs are situated in a membrane, they form a barrel-like structure permeable to retinoid ligands and control channel access to its binding sites. Once an ATP is hydrolized at the NBDs of the channel, NBDs are brought together to tilt and modify TMDs to modulate ligand binding to the channel. A recently proposed model of retinoid transfer occurring as a result of alternating exposure of external and internal TMD ligand binding sites, all controlled by binding of ATP, is based on recent structural analyses of bacterial ABC transporters.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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