Clues to the mechanism of cholesterol transfer from the structure of NPC1 middle lumenal domain bound to NPC2

Export of LDL-derived cholesterol from lysosomes requires the cooperation of the integral membrane protein Niemann–Pick C1 (NPC1) and a soluble protein, Niemann–Pick C2 (NPC2). Mutations in the genes encoding these proteins lead to Niemann–Pick disease type C (NPC). NPC2 binds to NPC1’s second (middle), lumenally oriented domain (MLD) and transfers cholesterol to NPC1’s N-terminal domain (NTD). Here, we report the 2.4-Å resolution crystal structure of a complex of human NPC1–MLD and NPC2 bearing bound cholesterol-3-O-sulfate. NPC1–MLD uses two protruding loops to bind NPC2, analogous to its interaction with the primed Ebola virus glycoprotein. Docking of the NPC1–NPC2 complex onto the full-length NPC1 structure reveals a direct cholesterol transfer tunnel between NPC2 and NTD cholesterol binding pockets, supporting the “hydrophobic hand-off” cholesterol transfer model.