Missense mutations in ABCG5 and ABCG8 disrupt heterodimerization and trafficking

Mutations in ABCG5 (G5) or ABCG8 (G8) cause sitosterolemia, an autosomal recessive disease characterized by sterol accumulation and premature atherosclerosis. G5 and G8 are ATP-binding cassette (ABC) half-transporters that must heterodimerize to move to the apical surface of cells. We examined the role of N-linked glycans in the formation of the G5/G8 heterodimer to gain insight into the determinants of folding and trafficking of these proteins. Site-directed mutagenesis revealed that two asparagine residues (Asn(585) and Asn(592)) are glycosylated in G5 and that G8 has a single N-linked glycan attached to Asn(619). N-Linked glycosylation of G8 was required for efficient trafficking of the G5/G8 heterodimer, but mutations that abolished glycosylation of G5 did not prevent trafficking of the heterodimer. Both G5 and G8 are bound by the lectin chaperone, calnexin, suggesting that the calnexin cycle may facilitate folding of the G5/G8 heterodimer. To determine the effects of 13 disease-causing missense mutations in G5 and G8 on formation and trafficking of the G5/G8 heterodimer, mutant forms of the half-transporters were expressed in CHO-K1 cells. All 13 mutations reduced trafficking of the G5/G8 heterodimer from the endoplasmic reticulum to the Golgi complex, and most prevented the formation of stable heterodimers between G5 and G8. We conclude that the majority of the molecular defects in G5 and G8 that cause sitosterolemia impair transport of the sterol transporter to the cell surface.