2.A.6.6.3
Yeast sterol transport system consisting of two proteins, NCR1 (YPL006w; 1170 aas) and NPC2 (YDL046w; ), components of the Niemann-Pick Type C transporter. It drives sterol integration into the lysosomal membrane before redistributing them to other cellular membranes. Expression of yeast NP-C-related gene 1 (NCR1), the orthologue of the human NP-C gene 1 (NPC1) defective in the disease, in Chinese hamster ovary NPC1 mutant cells suppressed lipid accumulation (Malathi et al. 2004). Deletion of NCR1, encoding a transmembrane glycoprotein predominantly residing in the vacuole of normal yeast, gave no phenotype. However, a dominant mutation in the putative sterol-sensing domain of Ncr1p conferred temperature and polyene antibiotic sensitivity without changes in sterol metabolism. Instead, the mutant cells were resistant to inhibitors of sphingolipid biosynthesis and super sensitive to sphingosine and C2-ceramide. Plasma membrane sphingolipids accumulated and redistributed to the vacuole and other subcellular membranes of the mutant cells. Malathi et al. 2004 proposed that the primordial function of these proteins is to recycle sphingolipids, and that defects in this process in higher eukaryotes secondarily result in cholesterol accumulation. Winkler et al. 2019 presented a framework for sterol membrane integration. Sterols are transferred between hydrophobic pockets of vacuolar NPC2 and NCR1. NCR1 has its N-terminal domain (NTD) positioned to deliver a sterol to a tunnel connecting the NTD to the luminal membrane leaflet, 50 Å away. A sterol is caught inside this tunnel during transport, and a proton-relay network of charged residues in the transmembrane region is linked to this tunnel, supporting a proton-driven transport mechanism. Winkler et al. 2019 proposed a model for sterol integration that clarifies the role of these NPC proteins.  Conformational changes in the Niemann-Pick type C1 protein NCR1 drive sterol translocation (Frain et al. 2024).  The membrane protein Niemann-Pick type C1 (NPC1, named NCR1 in yeast) is central to sterol homeostasis in eukaryotes. Saccharomyces cerevisiae NCR1 is localized to the vacuolar membrane carries sterols across the protective glycocalyx and deposits them into the vacuolar membrane.  Four cryo-EM structures of NCR1 in two distinct conformations, named tense and relaxed, have been described (Frain et al. 2024). These two conformations illustrate the movement of sterols through a tunnel formed by the luminal domains, thus bypassing the barrier presented by the glycocalyx. Based on these structures and on comparisons with other members of the RND superfamily, the authors proposed a transport model that links changes in the luminal domains with a cycle of protonation and deprotonation within the transmembrane region of the protein. This model suggests that NPC proteins work by a generalized RND mechanism where the proton motive force drives conformational changes in the transmembrane domains that are allosterically coupled to luminal/extracellular domains to promote sterol transport.  NCR1 has 1170 aas and 14 TMSs in a 1 (N-terminus) + 1 (residue 260) + 1 (residue 350) + 3 (residues 560 - 640) + 2 (residues 670 - 720) + 1 (residue 760) + 3 (residues 1100 - 1170) + 2 (residues 1100 - 1155). Iron limitation restores autophagy and increases lifespan in the yeast model of Niemann-Pick Type C1 (Martins et al. 2023). .