Stepwise phosphorylation of BLT1 defines complex assemblies with b-arrestin serving distinct functions.

Tatsumi,R., Aihara,S., Matsune,S., Aoki,J., Inoue,A., Shimizu,T., and Nakamura,M.

G-protein-coupled receptors (GPCRs) utilize complex cellular systems to respond to diverse ligand concentrations. By taking BLT1, a GPCR for leukotriene B4 (LTB4), as a model, our previous work elucidated that this system functions through the modulation of phosphorylation status on two specific residues: Thr308 and Ser310. Ser310 phosphorylation occurs at a lower LTB4 concentration than Thr308, leading to a shift in ligand affinity from a high-to-low state. However, the implications of BLT1 phosphorylation in signal transduction processes or the underlying mechanisms have remained unclear. Here, we identify the sequential BLT1-engaged conformations of b-arrestin and subsequent alterations in signal transduction. Stimulation of the high-affinity BLT1 with LTB4 induces phosphorylation at Ser310 via the ERK1/2-GRK pathway, resulting in a b-arrestin-bound low-affinity state. This configuration, referred to as the “low-LTB4-induced complex,” necessitates the finger loop region and the phosphoinositide-binding motif of b-arrestins to interact with BLT1 and deactivates the ERK1/2 signaling. Under high LTB4 concentrations, the low-affinity BLT1 again binds to the ligand and triggers the generation of the low-LTB4-induced complex into a different form termed “high-LTB4-induced complex.” This change is propelled by The308-phosphorylation-dependent basal-phosphorylation by PKCs. Within the high-LTB4-induced complex, b-arrestin adapts a unique configuration that involves additional N domain interaction to the low-affinity BLT1 and stimulates the PI3K/AKT pathway. We propose that the stepwise phosphorylation of BLT1 defines the formation of complex assemblies, wherein b-arrestins perform distinct functions.

The FASEB Journal

DOI:10.1096/fj.202301440R

DOI:10.1096/fj.202301440R