ABCG5 and ABCG8 form an obligate heterodimer that mediates Mg(2+)- and ATP-dependent sterol transport across the cell membrane. Plays an essential role in the selective transport of the dietary cholesterol in and out of the enterocytes and in the selective sterol excretion by the liver into bile. Required for normal sterol homeostasis. The heterodimer with ABCG5 has ATPase activity. ATP + cholesterol(in) + H2O = ADP + cholesterol(out) + H(+) + phosphate ATP + H2O + sitosterol(in) = ADP + H(+) + phosphate + sitosterol(out) Heterodimer with ABCG8. Highest expression in liver, with lower levels in small intestine and colon. A functional Walker motif (consensus sequence G-X-X-G-X-G-K-[ST]-T) is expected to bind ATP. The essential Lys in this region is not conserved in ABCG8 (G-S-S-G-C-R-A-S) and is not required for transport activity mediated by the heterodimer with ABCG5. N-glycosylated. N-glycosylation is important for efficient export out of the endoplasmic reticulum. Belongs to the ABC transporter superfamily. ABCG family. Eye pigment precursor importer (TC 3.A.1.204) subfamily. Seems to have a defective ATP-binding region.