Position 834 in TM6 plays an important role in cholesterol and phosphatidylcholine transport by ABCA1

Abstract

ATP-binding cassette protein A1 (ABCA1) plays a key role in eliminating excess cholesterol from peripheral cells by generating nascent high-density lipoprotein (HDL). However, it remains unclear whether both phospholipids and cholesterol are directly loaded onto apolipoprotein A-I (apoA-I) by ABCA1. To identify the amino acid residues of ABCA1 involved in substrate recognition and transport, we applied arginine scan mutagenesis to residues L821–E843 of human ABCA1 and predicted the environment to which each residue is exposed. The relative surface expression of each mutant suggested that residues L821–E843 pass through the plasma membrane as TM6, and the four residues (S826, F830, L834, and V837) of TM6 are exposed to the hydrophilic internal cavity of ABCA1. Furthermore, we showed that L834 is critical for the function of ABCA1.

Is ABCA1 a lipid transfer protein?

“…Mutagenesis has also been applied to investigate the effects of transmembrane helix structure on ABCA1 activity. Strikingly, the mutation L834R in TM helix 6 (Table 1) decreases PL and FC efflux without altering the amount of transporter in the PM (89). However, apoA-I binding is reduced presumably due to conformational changes in the ECDs caused by changes in the structure of TM helix 6 and its orientation in the PM.…”