Structural basis of sphingosine-1-phosphate receptor 1 activation and biased agonism

Sphingosine-1-phosphate receptor 1 (S1PR1) is a master regulator of lymphocyte egress from the lymph node and an established drug target for multiple sclerosis (MS). Mechanistically, therapeutic S1PR1 modulators activate the receptor yet induce sustained internalization through a potent association with β-arrestin. However, a structural basis of biased agonism remains elusive. Here, we report the cryo-electron microscopy (cryo-EM) structures of G_i-bound S1PR1 in complex with S1P, fingolimod-phosphate (FTY720-P) and siponimod (BAF312). In combination with functional assays and molecular dynamics (MD) studies, we reveal that the β-arrestin-biased ligands direct a distinct activation path in S1PR1 through the extensive interplay between the PIF and the NPxxY motifs. Specifically, the intermediate flipping of W269^6.48 and the retained interaction between F265^6.44 and N307^7.49 are the key features of the β-arrestin bias. We further identify ligand-receptor interactions accounting for the S1PR subtype specificity of BAF312. These structural insights provide a rational basis for designing novel signaling-biased S1PR modulators.