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A more recent version of this article appeared on March 1, 2006 Originally published as MBC in Press, 10.1091/mbc.E05-09-0899 on January 12, 2006
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Submitted on September 28, 2005
Revised on December 16, 2005
Accepted on January 3, 2006

Departments of *Pharmacology,
Cell and Developmental Biology, and
Genetics, School of Medicine, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599-7365
Monitoring Editor: Sandra Schmid
Sorting nexin 1 (SNX1) and SNX2 are the mammalian homologues of the yeast Vps5p retromer component that functions in endosome-to-Golgi trafficking. SNX1 is also implicated in endosome-to-lysosome sorting of cell surface receptors, although its requirement in this process remains to be determined. To assess SNX1 function in endocytic sorting of protease-activated receptor-1 (PAR1), we used siRNA to deplete HeLa cells of endogenous SNX1 protein. PAR1, a G-protein coupled receptor, is proteolytically activated by thrombin, internalized, sorted predominantly to lysosomes and efficiently degraded. Strikingly, depletion of endogenous SNX1 by siRNA markedly inhibited agonist-induced PAR1 degradation, whereas expression of a SNX1 siRNA resistant mutant protein restored agonist-promoted PAR1 degradation in cells lacking endogenous SNX1, indicating that SNX1 is necessary for lysosomal degradation of PAR1. SNX1 is known to interact with components of the mammalian retromer complex and Hrs, an early endosome associated protein. However, activated PAR1 degradation was not affected in cells depleted of retromer Vps26/Vps35 subunits, Hrs or Tsg101, an Hrs interacting protein. We further show that SNX2, which dimerizes with SNX1, is not essential for lysosomal sorting of PAR1, but rather it can regulate PAR1 degradation by disrupting endosomal localization of endogenous SNX1 when ectopically expressed. Together, our findings establish an essential role for endogenous SNX1 in sorting activated PAR1 to a distinct lysosomal degradative pathway that is independent of retromer, Hrs and Tsg101.
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