Review Intracellular trafficking of Niemann–Pick C proteins 1 and 2: obligate components of subcellular lipid transport Laura Liscum a, * , Stephen L. Sturley b a Department of Physiology, Tufts University School of Medicine, 136 Harrison, Avenue, Boston, MA 02111, United States b The Institute of Human Nutrition and Department of Pediatrics, Columbia University Medical Center, New York, NY 10032, United States Available online 1 September 2004 Abstract Niemann–Pick C 1 (NPC1) is a large integral membrane glycoprotein that resides in late endosomes, whereas NPC2 is a small soluble protein found in the lumen of lysosomes. Mutations in either NPC1 or NPC2 result in aberrant lipid transport from endocytic compartments, which results in lysosomal storage of a complex mixture of lipids, primarily cholesterol and glycosphingolipids. What are the biological functions of the NPC1 and NPC2 proteins? Here we review what is known about the intracellular itinerary of these two proteins as they facilitate lipid transport. We propose that the intracellular trafficking patterns of these proteins will provide clues about their function. D 2004 Elsevier B.V. All rights reserved. Keywords: Trafficking; Niemann–Pick C protein; Lipid 1. Introduction In normal cells, vesicular traffic from the plasma membrane through early endosomes and the endocytic recycling compartment brings fluid phase cargo (proteina- ceous or otherwise) and membrane constituents to late endosomes and lysosomes for digestion. The breakdown products are efficiently discharged from the late endosome/ lysosome (LE/L) compartment and reutilized by the cell’s biosynthetic machinery. However, in Niemann–Pick C (NPC) cells, cholesterol and glycosphingolipids accumulate in LE/L and their transfer to other intracellular destinations is very slow. The culprits in this transport blockage are the NPC1 and NPC2 proteins, which have long been speculated to be lysosomal proteins. Here we will examine the intracellular distribution of the functional NPC proteins and see how disease-causing mutations lead to changes in their itinerary. We will find that NPC2 can properly be called an LE/L resident, whereas NPC1 accrues significant frequent flyer miles as it shuttles around the cell. From these studies it is clear that, as in real estate, location is everything. 2. NPC1 and NPC2 in normal cells At steady state, NPC1 and NPC2 are found in late endosomes and lysosomes, respectively [1,2]. How do they get there? NPC1 and NPC2 proteins possess classical N- terminal signal peptides and are synthesized at the endoplas- mic reticulum (ER) and glycosylated at the ER and Golgi apparatus. NPC2, being a small soluble protein, likely follows the traditional route taken by most lysosomal hy- drolases to get to the LE/L compartment. The high mannose chain on NPC2 must be trimmed and phosphorylated because the protein binds the mannose-6-phosphate receptor (MPR) [2]. The MPR escorts NPC2 from the trans-Golgi network (TGN) to late endosomes, where it is released. MPR then cycles back to the TGN, whereas NPC2 is transferred to lysosomes [2,3]. This is a saturable process, and a significant amount of NPC2 protein escapes the lysosomes to be secreted. Tellingly, NPC2 was originally characterized as HE-1, a soluble 151-amino-acid lysosomal glycoprotein that is a major component of epididymal fluid [4]. The functional 1388-1981/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.bbalip.2004.08.008 * Corresponding author. Tel.: +1 617 636 6945; fax: +1 617 636 0445. E-mail address: laura.liscum@tufts.edu (L. Liscum). Biochimica et Biophysica Acta 1685 (2004) 22 – 27 www.bba-direct.com