GPCR kinases shape ACKR4 functions via differential C-terminal phosphorylation

Area of Science:

• Immunology
• Cell Biology
• Biochemistry

Background:

• Atypical chemokine receptor 4 (ACKR4) is a scavenger receptor influencing chemokine availability and G protein-coupled receptor (GPCR) signaling.
• Unlike classical chemokine receptors, ACKR4 exclusively signals through βarrestins, lacking G protein coupling.

Purpose of the Study:

• To elucidate the molecular mechanisms governing ACKR4 trafficking, βarrestin recruitment, and chemokine uptake.
• To investigate the role of ACKR4 phosphorylation by GPCR kinases (GRKs) in its function.

Main Methods:

• Analysis of ACKR4 steady-state trafficking and endosomal cycling.
• Identification of key serine and threonine residues in the ACKR4 tail region.
• Investigation of ACKR4 interactions with βarrestins and GRKs.
• Assessment of CCL19-mediated signaling and internalization.

Main Results:

• ACKR4 constitutively associates with βarrestins and cycles between cellular compartments.
• Specific serine/threonine clusters in the ACKR4 tail regulate trafficking, chemokine uptake, and βarrestin recruitment.
• ACKR4 is differentially phosphorylated by GRKs (GRK5/6 and GRK2/3) in a ligand-dependent manner.
• Apo ACKR4 forms a non-activating ternary complex with GRK2/3 and G protein.

Conclusions:

• The ACKR4 tail region, particularly C-terminal serine/threonine clusters, is critical for βarrestin recruitment and chemokine internalization.
• Differential GRK activity modulates ACKR4 phosphorylation and function.
• ACKR4's unique signaling bias and phosphorylation dynamics offer insights into chemokine system regulation.