ACKR3-arrestin2/3 complexes reveal molecular consequences of GRK-dependent barcoding

Area of Science:

• Structural biology
• Molecular pharmacology
• Cellular signaling

Background:

• Atypical chemokine receptor 3 (ACKR3/CXCR7) regulates CXCL12 levels and CXCR4 responsiveness.
• ACKR3 exclusively signals through arrestins, lacking G protein coupling.
• GRK2 and GRK5 differentially phosphorylate ACKR3, creating unique "barcodes" whose functional consequences remain unclear.

Approach:

• Cryo-electron microscopy (cryo-EM) was used to determine structures of ACKR3 bound to arrestin2 (Arr2) or arrestin3 (Arr3).
• ACKR3 was phosphorylated by either GRK2 or GRK5 to generate distinct phosphorylation patterns.
• A novel Fab fragment, Fab7, facilitated structure determination by binding both Arr2 and Arr3.

Key Points:

• Arrestin finger loops insert into the membrane/detergent, not the ACKR3 transmembrane core, unlike canonical GPCR-arrestin interactions.
• Phosphorylation site proximity to the ACKR3 core dictates arrestin binding mode: tighter for GRK5, heterogeneous/tail-focused for GRK2.
• Arr2 and Arr3 bind ACKR3 at different angles, influenced by C-terminal loop membrane interactions.

Conclusions:

• ACKR3's exclusive arrestin bias may stem from arrestins binding GRK-phosphorylated ACKR3 even when excluded from the binding pocket.
• Distinct phosphorylation barcodes and arrestin isoforms modulate ACKR3-arrestin complex configurations, potentially driving unique downstream signaling.
• The findings offer novel insights into GPCR-arrestin complex dynamics and ACKR3-specific signaling mechanisms.