Elevated conformational dynamics makes ACKR3 activation-prone and G protein-incompetent

Area of Science:

• Biochemistry
• Molecular Biology
• Structural Biology

Background:

• Chemokine receptors like CXCR4 and atypical receptors like ACKR3 mediate cell migration via CXCL12.
• CXCR4 signals through G proteins and β-arrestins, while ACKR3 is G protein-uncoupled and β-arrestin-biased.
• Differences in activation mechanisms and ligand sensitivity suggest distinct receptor dynamics between ACKR3 and CXCR4.

Purpose of the Study:

• To investigate the molecular basis for the distinct activation dynamics of ACKR3 and CXCR4.
• To compare the conformational changes of ACKR3 and CXCR4 upon ligand binding using molecular dynamics simulations.

Main Methods:

• Atomistic molecular dynamics (MD) simulations of ACKR3 and CXCR4 complexes with CXCL12 variants (WT and [P2G]).
• Analysis of transmembrane (TM) helix dynamics and residue interaction networks.
• Validation using Bioluminescence Resonance Energy Transfer (BRET)-based assays with receptor mutants.

Main Results:

• CXCR4 adopts a stable active state with WT CXCL12 but an inactive state with [P2G]CXCL12.
• ACKR3 displays variable TM6 and persistently active TM7 states with both WT and [P2G]CXCL12, retaining agonistic activity.
• Ligand-mediated networks regulating TM6/TM7 activation in CXCR4 are absent/disrupted in ACKR3, allowing less constrained dynamics.

Conclusions:

• ACKR3's unique conformational dynamics drive its activation propensity and ligand promiscuity.
• Distinct dynamics and disrupted interaction networks explain ACKR3's atypical signaling compared to CXCR4.
• These findings elucidate the structural basis for ACKR3's function in chemokine signaling.