The Processed Amino-Terminal Fragment of Human TLR7 Acts as a Chaperone To Direct Human TLR7 into Endosomes.
TLR7 mediates innate immune responses to viral RNA in endocytic compartments. Mouse and human (h)TLR7 undergo proteolytic cleavage, resulting in the generation of a C-terminal fragment that accumulates in endosomes and associates with the signaling adaptor MyD88 upon receptor triggering by TLR7 agonists. Although mouse TLR7 is cleaved in endosomes by acidic proteases, hTLR7 processing can occur at neutral pH throughout the secretory pathway through the activity of furin-like proprotein convertases. However, the mechanisms by which cleaved hTLR7 reaches the endosomal compartment remain unclear. In this study, we demonstrate that, after hTLR7 proteolytic processing, the liberated amino (N)-terminal fragment remains bound to the C terminus through disulfide bonds and provides key trafficking information that ensures correct delivery of the complex to endosomal compartments. In the absence of the N-terminal fragment, the C-terminal fragment is redirected to the cell surface, where it is functionally inactive. Our data reveal a novel role for the N terminus of hTLR7 as a molecular chaperone that provides processed hTLR7 with the correct targeting instructions to reach the endosomal compartment, hence ensuring its biological activity and preventing inadvertent cell surface responses to self-RNA.
MRC Human Immunology Unit, MRC Weatherall Institute of Molecular Medicine, Radcliffe Department of Medicine, University of Oxford, Oxford OX3 9DU, United Kingdom; and vincenzo.cerundolo@imm.ox.ac.uk madeleine.hipp@curevac.com.
Madeleine M
MM
Hipp
MRC Human Immunology Unit, MRC Weatherall Institute of Molecular Medicine, Radcliffe Department of Medicine, University of Oxford, Oxford OX3 9DU, United Kingdom; and.
Dawn
D
Shepherd
MRC Human Immunology Unit, MRC Weatherall Institute of Molecular Medicine, Radcliffe Department of Medicine, University of Oxford, Oxford OX3 9DU, United Kingdom; and.
Sarah
S
Booth
MRC Human Immunology Unit, MRC Weatherall Institute of Molecular Medicine, Radcliffe Department of Medicine, University of Oxford, Oxford OX3 9DU, United Kingdom; and.
Dominic
D
Waithe
The Francis Crick Institute, Lincoln's Inn Fields Laboratory, London WC2A 3LY, United Kingdom.
Caetano
C
Reis e Sousa
MRC Human Immunology Unit, MRC Weatherall Institute of Molecular Medicine, Radcliffe Department of Medicine, University of Oxford, Oxford OX3 9DU, United Kingdom; and vincenzo.cerundolo@imm.ox.ac.uk madeleine.hipp@curevac.com.
Vincenzo
V
Cerundolo
D002352:Carrier Proteins / Q000378:metabolism; D002460:Cell Line; D011992:Endosomes / Q000378:metabolism*; D057809:HEK293 Cells; D006801:Humans; D016209:Interleukin-8 / Q000378:metabolism; D018832:Molecular Chaperones / Q000378:metabolism*; D053594:Myeloid Differentiation Factor 88 / Q000378:metabolism; D010446:Peptide Fragments / Q000378:metabolism*; D043484:Proprotein Convertases / Q000378:metabolism; D011499:Protein Processing, Post-Translational / Q000235:genetics; D021381:Protein Transport / Q000502:physiology; D059748:Proteolysis; D051197:Toll-Like Receptor 4 / Q000235:genetics / Q000378:metabolism; D051199:Toll-Like Receptor 7 / Q000235:genetics* / Q000378:metabolism*
C508606:CXCL8 protein, human; D002352:Carrier Proteins; D016209:Interleukin-8; C507779:MYD88 protein, human; D018832:Molecular Chaperones; D053594:Myeloid Differentiation Factor 88; D010446:Peptide Fragments; C495345:TLR4 protein, human; C495348:TLR7 protein, human; D051197:Toll-Like Receptor 4; D051199:Toll-Like Receptor 7; D043484:Proprotein Convertases