A, Wild-type FFA4-S and FFA4-L (green insert), depicting the differences between the isoforms at the 3rd ICL. The β-arrestin-phosphosensor (blue) is comprised of acidic residues Glu341, Asp348, Asp355 (red) and the phosphorylation sites Thr347, Thr349, Ser350, Ser357, and Ser360 (yellow). B, FFA4 C-terminal mutants used in this study were truncated after Pro340 (corresponding to 356 in FFA4-L), thereby eliminating the C-terminal β-arrestin phosphosensor.