Rapid uptake and degradation of CXCL12 depend on CXCR7 carboxyl-terminal serine/threonine residues

J Biol Chem. 2012 Aug 17;287(34):28362-77. doi: 10.1074/jbc.M111.335679. Epub 2012 Jun 26.

Abstract

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.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Chemokine CXCL12 / genetics
  • Chemokine CXCL12 / metabolism*
  • HEK293 Cells
  • Humans
  • Mice
  • Mutation
  • Protein Structure, Tertiary
  • Protein Transport / physiology
  • Proteolysis*
  • Receptors, CXCR / genetics
  • Receptors, CXCR / metabolism*
  • Receptors, CXCR4 / genetics
  • Receptors, CXCR4 / metabolism

Substances

  • ACKR3 protein, human
  • CXCL12 protein, human
  • CXCR4 protein, human
  • CXCR4 protein, mouse
  • Chemokine CXCL12
  • Cmkor1 protein, mouse
  • Cxcl12 protein, mouse
  • Receptors, CXCR
  • Receptors, CXCR4