Phosphosite mapping was performed at the Purdue University Proteomics Facility. Briefly, ACKR3 in LMNG or nanodisc was first phosphorylated by GRK2 and GRK5 and then digested with trypsin. The fragments were analyzed via high-resolution MS without TiO2 enrichment, and phosphorylation sites identified through peptide ionization patterns compared to the non-phosphorylated primary amino acid sequence.