Research ArticleCHEMOKINE RECEPTORS

Functional anatomy of the full-length CXCR4-CXCL12 complex systematically dissected by quantitative model-guided mutagenesis

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Science Signaling  14 Jul 2020:
Vol. 13, Issue 640, eaay5024
DOI: 10.1126/scisignal.aay5024

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Illuminating CXCR4 activation

The small secreted protein CXCL12 and its cognate GPCR CXCR4 mediate cell migration and are implicated in immune cell trafficking, HIV-1 infection, and metastasis. Thus, structural analysis of the CXCR4-CXCL12 complex has implications for designing therapeutics. Stephens et al. tested their previous structural model of CXCR4-CXCL12 with a large panel of mutations and charged residue swapping experiments in cell-based functional assays and identified critical interactions across the large interface of the complex that are important for CXCR4 activation. These results challenge the established “two-site” hypothesis of chemokine receptor activation and provide structural insights into the mechanism of CXCR4 signaling.

Abstract

Because of their prominent roles in development, cancer, and HIV, the chemokine receptor CXCR4 and its ligand CXCL12 have been the subject of numerous structural and functional studies, but the determinants of ligand binding, selectivity, and signaling are still poorly understood. Here, building on our latest structural model, we used a systematic mutagenesis strategy to dissect the functional anatomy of the CXCR4-CXCL12 complex. Key charge swap mutagenesis experiments provided evidence for pairwise interactions between oppositely charged residues in the receptor and chemokine, confirming the accuracy of the predicted orientation of the chemokine relative to the receptor and providing insight into ligand selectivity. Progressive deletion of N-terminal residues revealed an unexpected contribution of the receptor N terminus to chemokine signaling. This finding challenges a longstanding “two-site” hypothesis about the essential features of the receptor-chemokine interaction in which the N terminus contributes only to binding affinity. Our results suggest that although the interaction of the chemokine N terminus with the receptor-binding pocket is the key driver of signaling, the signaling amplitude depends on the extent to which the receptor N terminus binds the chemokine. Together with systematic characterization of other epitopes, these data enable us to propose an experimentally consistent structural model for how CXCL12 binds CXCR4 and initiates signal transmission through the receptor transmembrane domain.

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