White pigment protein homolog of ABCG transporter subfamily
The White subfamily represents ABC transporters homologous to the Drosophila white gene, which acts as a dimeric importer for eye pigment precursors. The eye pigmentation of Drosophila is developed from the synthesis and deposition in the cells of red pigments, which are synthesized from guanine, and brown pigments, which are synthesized from tryptophan. The pigment precursors are encoded by the white, brown, and scarlet genes, respectively. Evidence from genetic and biochemical studies suggest that the White and Brown proteins function as heterodimers to import guanine, while the White and Scarlet proteins function to import tryptophan. However, a recent study also suggests that White may be involved in the transport of a metabolite, such as 3-hydroxykynurenine, across intracellular membranes. Mammalian ABC transporters belonging to the White subfamily (ABCG1, ABCG5, and ABCG8) have been shown to be involved in the regulation of lipid-trafficking mechanisms in macrophages, hepatocytes, and intestinal mucosa cells. ABCG1 (ABC8), the human homolog of the Drosophila white gene is induced in monocyte-derived macrophages during cholesterol influx mediated by acetylated low-density lipoprotein. It is possible that human ABCG1 forms heterodimers with several heterologous partners.