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Vol. 19, Issue 6, 2350-2362, June 2008

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Requirement of the Human GARP Complex for Mannose 6-phosphate-receptor-dependent Sorting of Cathepsin D to Lysosomes

Cell Biology and Metabolism Program, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, MD 20892

Submitted November 29, 2007; Revised February 26, 2008; Accepted March 19, 2008
Monitoring Editor: Jean Gruenberg

The biosynthetic sorting of acid hydrolases to lysosomes relies on transmembrane, mannose 6-phosphate receptors (MPRs) that cycle between the TGN and endosomes. Herein we report that maintenance of this cycling requires the function of the mammalian Golgi-associated retrograde protein (GARP) complex. Depletion of any of the three GARP subunits, Vps52, Vps53, or Vps54, by RNAi impairs sorting of the precursor of the acid hydrolase, cathepsin D, to lysosomes and leads to its secretion into the culture medium. As a consequence, lysosomes become swollen, likely due to a buildup of undegraded materials. Missorting of cathepsin D in GARP-depleted cells results from accumulation of recycling MPRs in a population of light, small vesicles downstream of endosomes. These vesicles might correspond to intermediates in retrograde transport from endosomes to the TGN. Depletion of GARP subunits also blocks the retrograde transport of the TGN protein, TGN46, and the B subunit of Shiga toxin. These observations indicate that the mammalian GARP complex plays a general role in the delivery of retrograde cargo into the TGN. We also report that a Vps54 mutant protein in the Wobbler mouse strain is active in retrograde transport, thus explaining the viability of these mutant mice.

Address correspondence to: Juan S. Bonifacino (juan{at}helix.nih.gov)

Abbreviations used: CatD, cathepsin D; MPR, mannose 6-phosphate receptor; CI-MPR, cation-independent MPR; TGN, trans-Golgi network; GARP, Golgi-associated retrograde protein; Vps, vacuolar protein sorting; STxB, B subunit of Shiga toxin; TEN, tubular endosomal network.