Sorting nexin-1 mediates tubular endosome-to-TGN transport through coincidence sensing of high- curvature membranes and 3-phosphoinositides.
BACKGROUND
Sorting nexins (SNXs) are phox homology (PX) domain-containing proteins thought to regulate endosomal sorting of internalized receptors. The prototypical SNX is sorting nexin-1 (SNX1), a protein that through its PX domain binds phosphatidylinositol 3-monophosphate [PtdIns(3)P] and phosphatidylinositol 3,5-bisphosphate [PtdIns(3,5)P(2)]. SNX1 is associated with early endosomes, from where it has been proposed to regulate the degradation of internalized epidermal growth factor (EGF) receptors through modulating endosomal-to-lysosomal sorting.


RESULTS
We show here that SNX1 contains a BAR (Bin/Amphiphysin/Rvs) domain, a membrane binding domain that endows SNX1 with the ability to form dimers and to sense membrane curvature. We present evidence that through coincidence detection, the BAR and PX domains efficiently target SNX1 to a microdomain of the early endosome defined by high curvature and the presence of 3-phosphoinositides. In addition, we show that the BAR domain endows SNX1 with an ability to tubulate membranes in-vitro and drive the tubulation of the endosomal compartment in-vivo. Using RNA interference (RNAi), we establish that SNX1 does not play a role in EGF or transferrin receptor sorting; rather it specifically perturbs endosome-to-trans Golgi network (TGN) transport of the cation-independent mannose-6-phosphate receptor (CI-MPR). Our data support an evolutionarily conserved function for SNX1 from yeast to mammals and provide functional insight into the molecular mechanisms underlying lipid-mediated protein targeting and tubular-based protein sorting.


CONCLUSIONS
We conclude that through coincidence detection SNX1 associates with a microdomain of the early endosome-characterized by high membrane curvature and the presence of 3-phosphoinositides-from where it regulates tubular-based endosome-to-TGN retrieval of the CI-MPR.
Carlton
Jez
J
Henry Wellcome Integrated Signalling Laboratories, Department of Biochemistry, School of Medical Sciences, University of Bristol, Bristol BS8 1TD, United Kingdom.
Bujny
Miriam
M
Peter
Brian J
BJ
Oorschot
Viola M J
VM
Rutherford
Anna
A
Mellor
Harry
H
Klumperman
Judith
J
McMahon
Harvey T
HT
Cullen
Peter J
PJ
D001692:Biological Transport; D002352:Carrier Proteins / Q000378:metabolism*; D011992:Endosomes / Q000378:metabolism*; D005455:Fluorescent Antibody Technique; D006367:HeLa Cells; D006801:Humans; D007425:Intracellular Membranes / Q000378:metabolism*; D008854:Microscopy, Electron; D010716:Phosphatidylinositols / Q000378:metabolism*; D017434:Protein Structure, Tertiary; D034622:RNA Interference; D034741:RNA, Small Interfering / Q000235:genetics; D017527:Receptor, IGF Type 2 / Q000378:metabolism; D014162:Transfection; D033921:Vesicular Transport Proteins / Q000378:metabolism*; D021601:trans-Golgi Network / Q000378:metabolism*
D002352:Carrier Proteins; D010716:Phosphatidylinositols; D034741:RNA, Small Interfering; D017527:Receptor, IGF Type 2; D033921:Vesicular Transport Proteins