Structural insights into CXCR4 modulation and oligomerization
Activation of the chemokine receptor CXCR4 by its chemokine ligand CXCL12 regulates diverse cellular processes. CXCR4 also serves as a key target for diseases such as cancer and HIV. Previously reported crystal structures of CXCR4 bound to antagonists revealed the architecture of an inactive, homodimeric receptor. However, many structural aspects of CXCR4 remain poorly understood, including its activation by CXCL12, as well as its assembly into higher-order oligomers. Here, we use cryoelectron microscopy (cryoEM) to investigate various modes of CXCR4 regulation in the presence and absence of Gi protein. CXCL12 activates CXCR4 by inserting its N-terminus deep into the CXCR4 orthosteric pocket. The binding of FDA-approved antagonist AMD3100 is stabilized by electrostatic interactions with acidic residues in the 7 transmembrane helix bundle. A potent antibody blocker, REGN7663, binds across the extracellular face of CXCR4 and inserts its CDR-H3 loop into the orthosteric pocket. Trimeric and tetrameric structures of CXCR4 reveal, to our knowledge, previously undescribed modes of GPCR oligomerization. Remarkably, CXCR4 adopts distinct subunit conformations in trimeric and tetrameric assemblies, highlighting how oligomerization could allosterically regulate chemokine receptor function.