SNX1 defines an early endosomal recycling exit for sortilin and mannose 6-phosphate receptors.
Mannose-6-phosphate receptors (MPRs) transport lysosomal hydrolases from the trans Golgi network (TGN) to endosomes. Recently, the multi-ligand receptor sortilin has also been implicated in this transport, but the transport carriers involved herein have not been identified. By quantitative immuno-electron microscopy, we localized endogenous sortilin of HepG2 cells predominantly to the TGN and endosomes. In the TGN, sortilin colocalized with MPRs in the same clathrin-coated vesicles. In endosomes, sortilin and MPRs concentrated in sorting nexin 1 (SNX1)-positive buds and vesicles. SNX1 depletion by small interfering RNA resulted in decreased pools of sortilin in the TGN and an increase in lysosomal degradation. These data indicate that sortilin and MPRs recycle to the TGN in SNX1-dependent carriers, which we named endosome-to-TGN transport carriers (ETCs). Notably, ETCs emerge from early endosomes (EE), lack recycling plasma membrane proteins and by three-dimensional electron tomography exhibit unique structural features. Hence, ETCs are distinct from hitherto described EE-derived membranes involved in recycling. Our data emphasize an important role of EEs in recycling to the TGN and indicate that different, specialized exit events occur on the same EE vacuole.
Mari
Muriel
M
Cell Microscopy Center, Department of Cell Biology, Institute of Biomembranes, University Medical Centre (UMC) Utrecht, AZU Rm G02.525, 3584 CX Utrecht, The Netherlands.
Bujny
Miriam V
MV
Zeuschner
Dagmar
D
Geerts
Willie J C
WJ
Griffith
Janice
J
Petersen
Claus M
CM
Cullen
Pete J
PJ
Klumperman
Judith
J
Geuze
Hans J
HJ
D033942:Adaptor Proteins, Vesicular Transport; D001483:Base Sequence; D002460:Cell Line; D011992:Endosomes / Q000378:metabolism*; D006801:Humans; D008562:Membrane Glycoproteins / Q000378:metabolism*; D016253:Microscopy, Immunoelectron; D008954:Models, Biological; D009419:Nerve Tissue Proteins / Q000378:metabolism*; D021381:Protein Transport; D034622:RNA Interference; D034741:RNA, Small Interfering / Q000235:genetics; D017527:Receptor, IGF Type 2 / Q000378:metabolism*; D058305:Sorting Nexins; D013347:Subcellular Fractions / Q000378:metabolism; D033921:Vesicular Transport Proteins / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism*; D021601:trans-Golgi Network / Q000378:metabolism; D000099183:Sortilin
D033942:Adaptor Proteins, Vesicular Transport; D008562:Membrane Glycoproteins; D009419:Nerve Tissue Proteins; D034741:RNA, Small Interfering; D017527:Receptor, IGF Type 2; D058305:Sorting Nexins; D033921:Vesicular Transport Proteins; D000099183:Sortilin; C513978:SNX1 protein, human