7 August 2017
The Niemann–Pick C1 (NPC1) protein is responsible for transporting LDL-derived cholesterol out of late endosomes. Mutations in NPC1 lead to the fatal Niemann–Pick Type C disease. We present here an improved structure of an NPC1 protein at 3.3 Å and decipher details of its C-terminal luminal domain (CTD), which could not be resolved in previous structures. In particular, a loop stabilized by a pair of disulfide bonds in the CTD binds to the N-terminal domain through a loop–loop interaction. We show that this interaction is important for cholesterol transport in cultured cells. Together, our data provide insights related to the molecular mechanism of NPC1 activity and Niemann–Pick Type C disease.
Niemann–Pick C1 (NPC1) and NPC2 proteins are indispensable for the export of LDL-derived cholesterol from late endosomes. Mutations in these proteins result in Niemann–Pick type C disease, a lysosomal storage disease. Despite recent reports of the NPC1 structure depicting its overall architecture, the function of its C-terminal luminal domain (CTD) remains poorly understood even though 45% of NPC disease-causing mutations are in this domain. Here, we report a crystal structure at 3.3 Å resolution of NPC1* (residues 314–1,278), which—in contrast to previous lower resolution structures—features the entire CTD well resolved. Notably, all eight cysteines of the CTD form four disulfide bonds, one of which (C909–C914) enforces a specific loop that in turn mediates an interaction with a loop of the N-terminal domain (NTD). Importantly, this loop and its interaction with the NTD were not observed in any previous structures due to the lower resolution. Our mutagenesis experiments highlight the physiological relevance of the CTD–NTD interaction, which might function to keep the NTD in the proper orientation for receiving cholesterol from NPC2. Additionally, this structure allows us to more precisely map all of the disease-causing mutations, allowing future molecular insights into the pathogenesis of NPC disease.