Localization of the murine Niemann-Pick C1 protein to two distinct intracellular compartments

Niemann-Pick type C (NPC) disease is characterized by an accumulation of ch olesterol and other lipids in the lysosomal compartment, In this report, we use subcellular fractionation and microscopy to determine the localization of the murine Niemann-Pick C1 (NPC1) protein. Fractionation of mouse liver homogenates indicates that some NPC1 cosediments with lysosome-associated membrane protein I (LAMP1)-containing membranes. However, a significant amo unt of NPC1 is also found in membranes that do not contain LAMP1. Moreover, fractionation of liver membranes and fibroblasts in the presence of a noni onic detergent showed that a fraction of NPC1 cosediments with caveolin-1 i n rafts. Immunofluorescence microscopy of cultured mouse fibroblasts showed that NPC1 is found in two morphologically distinct structures. The first p opulation is characterized by large punctate structures that do not colocal ize with major organelle protein markers, but do colocalize with filipin an d a small fraction of caveolin-1. Examination of these large NPC1-containin g compartments using electron microscopy shows that these structures contai n extensive internal membranes. The second population is represented by sma ller, more diffuse structures, a fraction of which colocalize with LAMP1-po sitive compartments. Incubation of fibroblasts with low density lipoprotein (LDL) increases colocalization of NPC1 with LAMP1-containing compartments. This colocalization can be further enhanced by treating fibroblasts with p rogesterone or chloroquine.jlr The results indicate that NPC1 is associated with an unique vesicular compartment enriched with cholesterol and con tai ning caveolin-1, and that NPC1 cycles to LAMP1-positive compartments, presu mably to facilitate the processing of LDL-derived cholesterol. Localization of the murine Niemann-Pick CI protein to two distinct intracellular compar tments.