Phosphorylation motif dictates GPCR C-terminal domain conformation and arrestin interaction

Lipids modulate the dynamics of GPCR:β-arrestin interaction

β-arrestins are key molecular partners of G Protein-Coupled Receptors (GPCRs), triggering not only their desensitization but also intracellular signaling. Existing structural data point to high conformational plasticity of GPCR:β-arrestin interaction, with two completely different orientations between receptor and β-arrestin. Combining molecular dynamics simulations and fluorescence spectroscopy, we show that β-arrestin 1 interacts with membranes even in the absence of a receptor, an interaction that is enhanced by PI(4,5)P2, presumably holding the β-arrestin 1 C-edge loop into the lipid bilayer. This key interaction helps β-arrestin 1 to adopt a "receptor-ready" orientation and consequently favors its coupling to the ghrelin receptor (GHSR). In addition, we show that the GHSR:β-arrestin 1 assembly is a dynamic complex where β-arrestin can adopt several orientations. PI(4,5)P2 decreases the dynamics of the complex and shifts the equilibrium between the different arrangements, favoring one of them. Taken together, our results highlight how PI(4,5)P2 plays a true third-player role in the GPCR:β-arrestin interaction, not only by preparing β-arrestin for its further interaction with receptors but also by modulating its orientation once the protein:protein complex is formed.

Action of the Terminal Complement Pathway on Cell Membranes

The complement pathway is one of the most ancient elements of the host's innate response and includes a set of protein effectors that rapidly react against pathogens. The late stages of the complement reaction are broadly categorised into two major outcomes. Firstly, C5a receptors, expressed on membranes of host cells, are activated by C5a to generate pro-inflammatory responses. Secondly, target cells are lysed by a hetero-oligomeric pore known as the membrane attack complex (MAC) that punctures the cellular membrane, causing ion and osmotic flux. Generally, several membrane-bound and soluble inhibitors protect the host membrane from complement damage. This includes inhibitors against the MAC, such as clusterin and CD59. This review addresses the most recent molecular and structural insights behind the activation and modulation of the integral membrane proteins, the C5a receptors (C5aR1 and C5aR2), as well as the regulation of MAC assembly. The second aspect of the review focuses on the molecular basis behind inflammatory diseases that are reflective of failure to regulate the terminal complement effectors. Although each arm is unique in its function, both pathways may share similar outcomes in these diseases. As such, the review outlines potential synergy and crosstalk between C5a receptor activation and MAC-mediated cellular responses.

The N-Linked Glycosylation Asn191 and Asn199 Sites Are Controlled Differently Between PKA Signal Transduction and pEKR1/2 Activity in Equine Follicle-Stimulating Hormone Receptor

Equine follicle-stimulating hormone receptor (eFSHR) contains four extracellular N-linked glycosylation sites, which play important roles in agonist-induced signal transduction. Glycosylation regulates G protein-coupled receptor mechanisms by influencing folding, ligand binding, signaling, trafficking, and internalization. Here, we examined whether the glycosylated sites in eFSHR are necessary for cyclic adenosine monophosphate (cAMP) signal transduction and the phosphate extracellular signal-regulated kinase 1/2 (pERK1/2) response. We constructed mutants (N191Q, N199Q, N268Q, and N293Q) of the four N-linked glycosylation sites in eFSHR using site-directed mutagenesis. In wild-type (wt) eFSHR, the cAMP response gradually increased dose-dependently, displaying a strong response at the EC50 and Rmax. Two mutants (N191Q and N199Q) considerably decreased the cAMP response. Both EC50 values were approximately 0.46- and 0.44-fold compared to that of the eFSHR-wt, whereas Rmax levels were 0.29- and 0.45-fold compared to eFSHR-wt because of high-ligand treatment. Specifically, the EC50 and Rmax values in the N268Q mutant were increased 1.23- and 1.46-fold, respectively, by eFSHR-wt. pERK1/2 activity in eFSHR-wt cells was rapid, peaked within 5 min, consistently sustained until 15 min, and then sharply decreased. pERK1/2 activity in the N191Q mutant showed a pattern similar to that of the wild type, despite impaired cAMP responsiveness. The N199Q mutant showed low pERK1/2 activity at 5 and 15 min. Interestingly, pERK1/2 activity in the N268Q and N298Q mutants was similar to that of eFSHR-wt at 5 min, but neither mutant showed any signaling at 15 min, despite displaying high cAMP responsiveness. Overall, eFSHR N-linked glycosylation sites can signal to pERK1/2 via PKA and the other signals, dependent on G protein coupling and β-arrestin-dependent recruitment. Our results provide strong evidence for a new paradigm in which cAMP signaling is not activated, yet pERK1/2 cascade remains strongly induced.

Signal Transduction of Constitutive Activating and Inactivating Eel Lutropin/Choriogonadotropin Receptor (Eel LH/CGR) Mutants by Recombinant Equine Chorionic Gonadotropin (Rec-eCG)

Lutropin/choriogonadotropin receptor (LH/CGR) is a member of the G protein-coupled receptor superfamily. LH/CGRs in fish and mammalian species have been reported to contain naturally occurring, constitutively activating, and inactivating mutations in highly conserved regions. The present study was designed to determine the functional aspect of eel LH/CGR signal transduction. Biochemical analysis was performed using cells transfected with wild-type eel LH/CG (eel LH/CGR-wt) or with activating (designated eel LH/CGR-M410T, L469R, and D590Y) and inactivating (eel LH/CGR-D 417N and Y558F) mutants. We also generated a mutant (eel LH/CGR-t651) in which the C-terminal cytoplasmic tail was truncated at residue 651. Activating mutant cells expressing eel LH/CGR-M410T, L469R, and D590Y exhibited 1.4-, 8.7-, and 4.0-fold increases in the basal cAMP response, respectively, without recombinant equine chorionic gonadotropin (rec-eCG) agonist treatment. In inactivating mutants (eel LH/CGR-D417N and Y558F), the cyclic adenosine monophosphate (cAMP) response did not result in completely impaired signal transduction. However, the eel LH/CGR-t651 mutant did not exhibit any cAMP signaling following high-agonist treatment. Rmax values did not increase with further rec-eCG agonist stimulation. Our results suggest that constitutively activating and inactivating eel LH/CGR mutants with highly conserved amino acids exhibit a significant signal transduction pathway for glycoprotein hormone receptors. Eel LH/CGRs in activating and inactivating mutants are usually processed by receptor-mediated signaling following rec-eCG agonist stimulation.

Characterization of Trileucine Motif in the C-Terminus of the Equine Lutropin/Choriogonadotropin Receptor

The lutropin/chorionic gonadotropin receptor (LH/CGR) belongs to the G protein-coupled receptor family, characterized by conserved leucine residues in their carboxyl-terminal cytoplasmic tails. This study aimed to investigate the functional significance of the equine LH/CGR (eLH/CGR) trileucine motif in signal transduction. Wild-type eLH/CGR (eLH/CGR-wt) and mutant receptors, in which the trileucine motif was altered to alanine (eLH/CGR-ALL, LAL, LLA, and AAA), were analyzed in transfected cells. The expression levels of mutants ranged from 60% to 78%, with eLH/CGR-AAA showing the lowest level. Although the trileucine motif did not individually affect cAMP responsiveness, the combined mutant (eLH/CGR-AAA) significantly reduced cAMP response, surface receptor levels and enhanced receptor internalization rates. Activation of phospho-ERK1/2 was rapid in all mutants, peaking at 5 min, but eLH/CGR-ALL and LAL mutants exhibited a sharp decline in activity at 15 min. Notably, the eLH/CGR-LLA and AAA mutants showed similar phospho-ERK1/2 activity as the wild type. The eLH/CGR-AAA mutant also displayed a two-fold reduction in PKA signal transduction. These findings suggest that while individual leucine residues of the trileucine motif do not affect cAMP responsiveness, the entire motif plays a crucial role in receptor trafficking and signaling, specifically influencing PKA and phospho-ERK1/2 pathways.

A Structural Mechanism for Noncanonical GPCR Signal Transduction in the Hedgehog Pathway

The Hedgehog (Hh) signaling pathway is fundamental to embryogenesis, tissue homeostasis, and cancer. Hh signals are transduced via an unusual mechanism: upon agonist-induced phosphorylation, the noncanonical G protein-coupled receptor SMOOTHENED (SMO) binds the catalytic subunit of protein kinase A (PKA-C) and physically blocks its enzymatic activity. By combining computational structural approaches with biochemical and functional studies, we show that SMO mimics strategies prevalent in canonical GPCR and PKA signaling complexes, despite little sequence or secondary structural homology. An intrinsically disordered region of SMO binds the PKA-C active site, resembling the PKA regulatory subunit (PKA-R) / PKA-C holoenzyme, while the SMO transmembrane domain binds a conserved PKA-C interaction hub, similar to other GPCR-effector complexes. In contrast with prevailing GPCR signal transduction models, phosphorylation of SMO promotes intramolecular electrostatic interactions that stabilize key structural elements within the SMO cytoplasmic domain, thereby remodeling it into a PKA-inhibiting conformation. Our work provides a structural mechanism for a central step in the Hh cascade and defines a paradigm for disordered GPCR domains to transmit signals intracellularly.

Control of G protein-coupled receptor function via membrane-interacting intrinsically disordered C-terminal domains

G protein-coupled receptors (GPCRs) control intracellular signaling cascades via agonist-dependent coupling to intracellular transducers including heterotrimeric G proteins, GPCR kinases (GRKs), and arrestins. In addition to their critical interactions with the transmembrane core of active GPCRs, all three classes of transducers have also been reported to interact with receptor C-terminal domains (CTDs). An underexplored aspect of GPCR CTDs is their possible role as lipid sensors given their proximity to the membrane. CTD-membrane interactions have the potential to control the accessibility of key regulatory CTD residues to downstream effectors and transducers. Here, we report that the CTDs of two closely related family C GPCRs, metabotropic glutamate receptor 2 (mGluR2) and mGluR3, bind to membranes and that this interaction can regulate receptor function. We first characterize CTD structure with NMR spectroscopy, revealing lipid composition-dependent modes of membrane binding. Using molecular dynamics simulations and structure-guided mutagenesis, we then identify key conserved residues and cancer-associated mutations that modulate CTD-membrane binding. Finally, we provide evidence that mGluR3 transducer coupling is controlled by CTD-membrane interactions in live cells, which may be subject to regulation by CTD phosphorylation and changes in membrane composition. This work reveals an additional mechanism of GPCR modulation, suggesting that CTD-membrane binding may be a general regulatory mode throughout the broad GPCR superfamily.

Ready for the sheet: β-strand folding of phosphorylation clusters guides GPCR binding to arrestin

The molecular dynamics of arrestin binding to G protein-coupled receptors (GPCRs) are still poorly understood. In this issue of Structure, Guillien et al. show that negative charges in GPCR key phosphorylation clusters induce the formation of a transient β-strand that participates in an intermolecular β-sheet in the associated complex.