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Vol. 18, Issue 3, 768-780, March 2007

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BLOC-1 Is Required for Cargo-specific Sorting from Vacuolar Early Endosomes toward Lysosome-related Organelles

Subba Rao Gangi Setty^*, Danièle Tenza, Steven T. Truschel^*, Evelyn Chou^*, Elena V. Sviderskaya, Alexander C. Theos^*, M. Lynn Lamoreux, Santiago M. Di Pietro^||, Marta Starcevic^||, Dorothy C. Bennett, Esteban C. Dell'Angelica^||, Graça Raposo, and Michael S. Marks^*

*Department of Pathology and Laboratory Medicine, University of Pennsylvania, Philadelphia, PA 19104; Institut Curie, Centre National de la Recherche Scientifique-Unité Mixte de Recherche 144, Paris 75248, France; Department of Basic Medical Sciences, St. George's Hospital Medical School, London, SW17 ORE, United Kingdom; Comparative Genetics Program, Texas A&M University, College Station, TX 77843; and ^||Department of Human Genetics, University of California, Los Angeles, Los Angeles, CA 90095

Submitted December 4, 2006; Accepted December 7, 2006
Monitoring Editor: Sandra Schmid

Hermansky-Pudlak syndrome (HPS) is a genetic disorder characterized by defects in the formation and function of lysosome-related organelles such as melanosomes. HPS in humans or mice is caused by mutations in any of 15 genes, five of which encode subunits of biogenesis of lysosome-related organelles complex (BLOC)-1, a protein complex with no known function. Here, we show that BLOC-1 functions in selective cargo exit from early endosomes toward melanosomes. BLOC-1–deficient melanocytes accumulate the melanosomal protein tyrosinase-related protein-1 (Tyrp1), but not other melanosomal proteins, in endosomal vacuoles and the cell surface due to failed biosynthetic transit from early endosomes to melanosomes and consequent increased endocytic flux. The defects are corrected by restoration of the missing BLOC-1 subunit. Melanocytes from HPS model mice lacking a different protein complex, BLOC-2, accumulate Tyrp1 in distinct downstream endosomal intermediates, suggesting that BLOC-1 and BLOC-2 act sequentially in the same pathway. By contrast, intracellular Tyrp1 is correctly targeted to melanosomes in melanocytes lacking another HPS-associated protein complex, adaptor protein (AP)-3. The results indicate that melanosome maturation requires at least two cargo transport pathways directly from early endosomes to melanosomes, one pathway mediated by AP-3 and one pathway mediated by BLOC-1 and BLOC-2, that are deficient in several forms of HPS.

The online version of this article contains supplemental material at MBC Online (http://www.molbiolcell.org).

Address correspondence to: Michael S. Marks (marksm{at}mail.med.upenn.edu)

Abbreviations used: BLOC, biogenesis of lysosome-related organelles complex; HPS, Hermansky-Pudlak syndrome; IEM, immunoelectron microscopy; IFM, immunofluorescence microscopy; LRO, lysosome-related organelle; MuHA, HA11-epitope tagged muted; PaHA, HA11-epitope tagged pallidin; rp, reduced pigmentation; syn13, syntaxin 13; Tf, transferrin; TfR, transferrin receptor; Tyrp1, tyrosinase-related protein-1.

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