Introduction of ABCG5
ABCG5, ATP-binding cassette subfamily G member 5 is a protein that in humans is encoded by the ABCG5 gene. It is a member of the superfamily of ATP-binding cassette (ABC) transporters. ABC proteins transport various molecules across extra- and intra-cellular membranes. ABC genes are divided into seven distinct subfamilies (ABC1, MDR/TAP, MRP, ALD, OABP, GCN20, White). This protein is a member of the White subfamily. The protein encoded by this gene functions as a half-transporter to limit intestinal absorption and promotes biliary excretion of sterols. It is expressed in a tissue-specific manner in the liver, colon, and intestine.
|Basic Information of ABCG5|
|Protein Name||ATP-binding cassette sub-family G member 5|
|Organism||Homo sapiens (Human)|
Function of ABCG5 Membrane Protein
ABCG5, ATP-binding cassette sub-family G member 5 belongs to the ABCG subfamily, the members of which are half transporters composed of a single trans-membrane and a single nucleotide-binding domain. This gene is tandemly arrayed on chromosome 2, in a head-to-head orientation with family member ABCG8. Mutations in this gene may contribute to sterol accumulation and atherosclerosis, and have been observed in patients with sitosterolemia. ABCG5 and ABCG8 form an obligate heterodimer that mediates Mg2+ and ATP-dependent sterol transport across the cell membrane. It plays an essential role in the selective transport of dietary plant sterols and cholesterol in and out of the enterocytes and in the selective sterol excretion by the liver into bile.
Fig.1 Structure of the G5G8 heterodimer. (Lee, 2016)
Application of ABCG5 Membrane Protein in Literature
This article used crystallization in lipid bilayers to determine the X-ray structure of human G5G8 in a nucleotide-free state at 3.9 Å resolution, generating the first atomic model of an ABC sterol transporter. The G5G8 structure provided a mechanistic framework for understanding sterol transport and the disruptive effects of mutations causing sitosterolemia.
This article identified a binding site for hepatocyte nuclear factor 4alpha (HNF4alpha) in the ABCG5/ABCG8 intergenic promoter, through which HNF4alpha strongly activated the expression of a reporter gene in both directions. It was the first demonstration that HNF4alpha acted synergistically with GATA factors to activate gene expression in a bidirectional fashion.
This study examined the roles of the Walker A, Walker B, and signature motifs in the nucleotide-binding domains (NBD) of ABCG5 and ABCG8 using recombinant adenoviruses to reconstitute biliary sterol transport in G5G8-deficient mice. It also indicated that the NBDs of G5 and G8 were not functionally equivalent.
This article showed that the heterodimeric units were physically and biologically inseparable in their journey to the apical membrane, also the molecular, biological, biochemical, and biophysical results of flooding bile with enormous amounts of excess cholesterol that was, in part, prevented from being reabsorbed.
These studies indicated that both ABCG5 and ABCG8 were indispensable for the regulation of sterol absorption and excretion. Identification of ABCG5 and ABCG8 as critical players in the regulation of dietary-sterol absorption and excretion identified a new pathway of sterol transport.
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