Rapid uptake and degradation of CXCL12 depend on CXCR7 carboxyl-terminal serine/threonine residues.
CXCL12 signaling through G protein-coupled CXCR4 regulates cell migration during ontogenesis and disease states including cancer and inflammation. The second CXCL12-receptor CXCR7 modulates the CXCL12/CXCR4 pathway by acting as a CXCL12 scavenger and exerts G protein-independent functions. Given the distinct properties of CXCR4 and CXCR7, we hypothesized that the distinct C-terminal domains differently regulate receptor trafficking and stability. Here, we examined epitope-tagged wild type and C-terminal mutant receptors in human embryonic kidney cells (HEK293) with respect to trafficking, stability, (125)I-CXCL12 degradation, and G protein-coupling. The 24 CXCR7 C-terminal residues were sufficient to promote rapid spontaneous internalization. Replacement of the CXCR7 C terminus with that of CXCR4 (CXCR7-4tail mutant) abolished spontaneous internalization but permitted ligand-induced internalization and phosphorylation at the heterologous domain. The reverse tail-swap caused ligand-independent internalization of the resulting CXCR4-7tail mutant. Receptor-mediated (125)I-CXCL12 uptake and release of (125)I-CXCL12 degradation products were accelerated with receptors bearing the CXCR7 C terminus and impaired after conversion of CXCR7 C-terminal serine/threonine residues into alanines. C-terminal lysine residues were dispensable for plasma membrane targeting and the CXCL12 scavenger function but involved in constitutive degradation of CXCR7. Although the CXCR7 C terminus abolished G protein coupling in the CXCR4-7tail mutant, replacement of the CXCR7 C terminus, CXCR7 second intracellular loop, or both domains with the corresponding CXCR4 domain did not result in a G protein-coupled CXCR7 chimera. Taken together, we provide evidence that the CXCR7 C terminus influences the ligand-uptake/degradation rate, G protein coupling, and receptor stability. Regulatory pathways targeting CXCR7 C-terminal serine/threonine sites may control the CXCL12 scavenger activity of CXCR7.
Institute of Pharmacology and Toxicology, Jena University Hospital-Friedrich Schiller University Jena, Jena, Germany.
Frauke
F
Hoffmann
Wiebke
W
Müller
Dagmar
D
Schütz
Mark E
ME
Penfold
Yung H
YH
Wong
Stefan
S
Schulz
Ralf
R
Stumm
D000818:Animals; D054377:Chemokine CXCL12 / Q000235:genetics / Q000378:metabolism*; D057809:HEK293 Cells; D006801:Humans; D051379:Mice; D009154:Mutation; D017434:Protein Structure, Tertiary; D021381:Protein Transport / Q000502:physiology; D059748:Proteolysis; D054387:Receptors, CXCR / Q000235:genetics / Q000378:metabolism*; D019718:Receptors, CXCR4 / Q000235:genetics / Q000378:metabolism
C501896:ACKR3 protein, human; C515255:CXCL12 protein, human; C527276:CXCR4 protein, human; C514936:CXCR4 protein, mouse; D054377:Chemokine CXCL12; C501897:Cmkor1 protein, mouse; C515256:Cxcl12 protein, mouse; D054387:Receptors, CXCR; D019718:Receptors, CXCR4