Calreticulin facilitates the cell surface expression of ABCG5/G8

ATP-binding cassette (ABC) G5 (G5) and ABCG8 (G8) heterodimerize and function as sterol transporter that promote biliary excretion of neutral sterols. Both G5 and G8 interact with a lectin-like chaperone, calnexin (CNX), in the endoplasmic reticulum (ER) but the significance of this interaction remains unclear. Here, we show that not only CNX, but also its homologue calreticulin (CRT), is involved in the biosynthesis of G5/G8 sterol transporter. Both CNX and CRT interacted with immature forms of G5 and G8, and stimulated their productive folding by inhibiting their degradation. Interestingly, CRT predominantly enhanced the cell surface expression of mature G5/G8 whereas CNX did not have a similar effect. Inhibitors of N-glycan processing indicated that quality control of G5 and G8 might be differentially regulated in the ER. These findings clarify the role of CNX and CRT in the biosynthesis and quality control of G5/G8 sterol transporter.