Ligand-independent higher-order multimerization of CXCR4, a G-protein-coupled chemokine receptor involved in targeted metastasis.

CXCR4, a G-protein-coupled receptor of CXCL12/stromal cell-derived factor-1alpha, mediates a wide range of physiological and pathological processes, including the targeted metastasis of cancer cells. CXCR4 has been shown to homo-oligomerize in several experimental systems. However, it remains unclear with which domains CXCR4 interacts homotypically, and whether it dimerizes or forms a higher-order complex. To address these issues, we used bioluminescent resonance energy transfer and bimolecular fluorescence complementation analyses to measure the homotypic interactions of CXCR4 in living cells. Both assays indicated that CXCR4 interacts homotypically, which is consistent with previous studies. By studying CXCR4 mutants lacking various domains, we found that multiple transmembrane domains probably serve as potential molecular interaction surfaces for oligomerization. The relative contribution of the amino- or carboxy-termini to oligomerization was small. To differentiate between a dimer and a multimer consisting of more than two molecules, bioluminescent resonance energy transfer-bimolecular fluorescence complementation analysis was conducted. It revealed that CXCR4 engages in higher-order oligomerization in a ligand-independent fashion. This is the first report providing direct experimental evidence for the higher-order multimerization of CXCR4 in vivo. We hypothesize that CXCR4 distributes to the cell surface as a multimer, in order to effectively sense, with increased avidity, the chemotaxis-inducing ligand in the microenvironment. Studying the structure and function of the oligomeric state of CXCR4 may lead us to develop novel CXCR4 inhibitors that disassemble the molecular cluster of CXCR4.