Rapid Uptake and Degradation of CXCL12 Depend on CXCR7 Carboxyl-terminal Serine/Threonine Residues

• Published in: The Journal of biological chemistry 2012-08, Vol.287 (34), p.28362-28377
• Main Authors: Hoffmann, Frauke, Müller, Wiebke, Schütz, Dagmar, Penfold, Mark E., Wong, Yung H., Schulz, Stefan, Stumm, Ralf
• Format: Article
• Language: English
• Subjects: Animals; Calcium; Cell Biology; Chemokine CXCL12 - genetics; Chemokine CXCL12 - metabolism; Chemokines; CXCL12; Cxcr4; CXCR7; Decoy Receptor; G Protein-coupled Receptors (GPCR); HEK293 Cells; Humans; Mice; Mutation; Protein Structure, Tertiary; Protein Transport - physiology; Proteolysis; Receptor Endocytosis; Receptors, CXCR - genetics; Receptors, CXCR - metabolism; Receptors, CXCR4 - genetics; Receptors, CXCR4 - metabolism; Scavenger; SDF-1
• ISSN: 0021-9258; 1083-351X
• DOI: 10.1074/jbc.M111.335679

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, 125I-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 125I-CXCL12 uptake and release of 125I-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. Background: CXCR7 is an atypical heptahelical receptor that functions as scavenger for the endogenous chemokine ligand but fails to signal through G-proteins. Results: The CXCR7 C terminus causes uncoupling from G-protein, rapid receptor-mediated ligand uptake, and constitutive receptor degradation. Conclusion: Atypical CXCR7 functions are encoded in C-terminal elements. Significance: Identification of structural determinants regulating atypical and canonical functions of heptahelical receptors.