ABSTRACT
Specific G-protein-coupled receptors (GPCRs) exist on the ciliary membrane. Hedgehog signaling activation triggers the import of Smoothened into and export of GPR161 from cilia. The BBSome, which comprises eight Bardet–Biedl syndrome (BBS) proteins, mediates GPCR export, together with the intraflagellar transport (IFT) machinery, containing the IFT-A and IFT-B complexes. The absence of any BBSome subunit or IFT27 (also known as BBS19) (an IFT-B subunit) impairs ciliary GPCR export, including that of GPR161. Plasma membrane GPCRs undergo phosphorylation by GPCR kinases (GRKs) and subsequent binding of β-arrestins [β-arrestin1 (ARRB1) and β-arrestin2 (ARRB2)], which is crucial for clathrin-mediated endocytosis. We here confirmed that GPR161 and β-arrestin are accumulated within cilia in the absence of IFT27 or the BBSome, and that ARRB1 and ARRB2 double-knockout impairs GPR161 export. Notably, we found that activation-mimetic β-arrestin mutants can interact with both the BBSome and ciliary GPCRs, and cause constitutive export of GPR161. Moreover, we demonstrated that GRK2 plays a crucial role in GPR161 export. We here propose that phosphorylated GPR161 recruits β-arrestins, converting them into their activated conformation. Activated β-arrestins then interact with the BBSome, which connects them to the IFT machinery to facilitate GPR161 export.
Footnotes
Author contributions
Conceptualization: T.F., Y.K., K.N.; Data curation: T.F., N.M.; Formal analysis: T.F., N.M.; Funding acquisition: Y.K., K.N.; Investigation: T.F., N.M., S.A.; Methodology: H.T.; Project administration: K.N.; Supervision: H.-W.S., Y.K., K.N.; Validation: T.F., N.M., K.N.; Writing – original draft: K.N.; Writing – review & editing: T.F., N.M., H.T., H.-W.S., Y.K., K.N.
Funding
This work was supported in part by grants from the Japan Society for the Promotion of Science (grant numbers 20H04904 and 24K02022 to K.N., and 21H02427 and 22H05539 to Y.K.), and from Takeda Science Foundation to Y.K.
Data and resource availability
All relevant data can be found within the article and its supplementary information.
Special Issue
This article is part of the Special Issue ‘Cilia and Flagella: from Basic Biology to Disease’, guest edited by Pleasantine Mill and Lotte Pedersen. See related articles at https://journals.biologists.com/jcs/issue/138/20.