A Membrane-proximal, C-terminal α-Helix Is Required for Plasma Membrane Localization and Function of the G Protein-coupled Receptor (GPCR) TGR5

Abstract

Background: TGR5 is a G protein-coupled bile acid receptor that modulates the immune response, glucose homeostasis, and liver regeneration. Results: Secondary structure of the receptor C terminus determines plasma membrane trafficking. Conclusion: TGR5 plasma membrane content and responsiveness to extracellular ligands depends on C-terminal ␣-helix formation.Significance: This provides insights into the structure-function relationship of TGR5, which is a potential drug target for metabolic diseases.

Structural assemblies of the di- and oligomeric G-protein coupled receptor TGR5 in live cells: an MFIS-FRET and integrative modelling study

“…In fact, the MFIS-FRET measurements showed no change in FRET properties after TC treatment, an observation that is also supported by literature22. As an indicator of G-protein binding, we successfully proved cAMP increase after ligand treatment in all TGR5 variants, which has also been shown recently94243. We have no evidence that TGR5 oligomerization is affected by ligand treatment and subsequent G-protein coupling.…”

“…This has just been demonstrated for rhodopsin where disruption of dimerization with small peptides decreased receptor stability44. We recently showed that the loss of α-helicality in the TGR5 C-terminus, which constitutes the major interaction surface in the 1/8 interface, severely impairs TGR5 membrane localization and activity9. One can thus speculate that this influence on membrane localization and activity results from a distorted TGR5 dimerization in the ER.…”

“…1a,b). Furthermore, TGR5 responsiveness towards TLC was investigated using a cAMP-responsive luciferase assay9, where luciferase activity served as a measure for the second messenger cAMP following TGR5 activation. Forskolin (F) elevates cAMP independent of TGR5 and was used as positive control.…”

“…Homology models of TGR5 have been presented based on template structures of other seven transmembrane (7TM) domain receptors5678. We previously reported that the amino acids 285–294 at the TGR5 C-terminus form an alpha-helical stretch important for plasma membrane localization and thus responsiveness to extracellular ligands9.…”

The secondary structure of the TGR5 membrane-proximal C-terminus determines plasma membrane localization and responsiveness towards extracellular ligands

“…The membrane proximal C-terminus of GPCRs has been shown to be important for membrane localization of these receptors because it can contain a sorting motif (e.g., the F(X) 6 LL motif). Since the N-terminal part of the C-Terminus of TGR5 contains no known sorting motif, we hypothesize that it must be the secondary structure of the C-terminus that determines TGR5 trafficking [3]. …”