Different contributions of chemokine N‐terminal features attest to a different ligand binding mode and a bias towards activation of ACKR3/CXCR7 compared with CXCR4 and CXCR3

Abstract

Chemokines and their receptors form an intricate interaction and signaling network that plays critical roles in various physiological and pathological cellular processes. The high promiscuity and apparent redundancy of this network makes probing individual chemokine/receptor interactions and functional effects, as well as targeting individual receptor axes for therapeutic applications, challenging. Despite poor sequence identity, the N-terminal regions of chemokines, which play a key role in their activity and selectivity, harbor several conserved features. Thus far, little is known regarding the molecular basis of their interactions with conventional vs. atypical chemokine receptors or the conservation of their contributions across chemokine-receptor pairs. In this study, using a broad panel of chemokine variants and modified peptides derived from the N-terminal region of chemokines CXCL12, CXCL11, and vCCL2, we compare the role of various features in binding and activation of their shared receptors, the two canonical G protein-signaling receptors, CXCR4 and CXCR3, as well as the atypical scavenger receptor CXCR7/ACKR3, which shows exclusively arrestin-dependent activity. We provide exhaustive molecular insights into the plasticity of the ligand-binding pockets of these receptors, their chemokine binding modes, and their activation mechanisms. We show that, although the chemokine N-terminal region is a critical determinant, neither the most proximal residues nor the N-loop are essential for ACKR3 binding and activation, as opposed to CXCR4 and CXCR3. These results suggest a different interaction mechanism between this atypical receptor and its ligands and illustrates its strong propensity to activation.