Exploring the activation mechanism of metabotropic glutamate receptor 2

Triple Labeling Resolves a GPCR Intermediate State by Using Three-Color Single Molecule FRET

The correlation of individual conformational changes in dynamic protein complexes remains challenging as most structural methods rely on averaged information over a large number of molecules. Single molecule FRET is a powerful tool for monitoring such conformational changes. When performed using three distinct probes, it enables the correlation of domain movements by providing up to three simultaneous distance measurements with high temporal resolution. Nevertheless, a major challenge lies in the site-specific attachment of three probes to unique positions within the target protein. Here, we propose an orthogonal triple-labeling strategy that is not compromised by native, reactive amino acid functionalities. It combines genetic code expansion and bioorthogonal labeling of two different noncanonical amino acids with an enzymatic self-labeling SNAP tag. We demonstrate its application by establishment of a 3-color sensor on the human metabotropic glutamate receptor 2, a dimeric, multidomain G protein-coupled neuroreceptor, and describe a previously unknown conformational intermediate state using 3-color single molecule FRET.

Novel 6-Aryl isoindolin-1-one Derivatives as Negative Allosteric Modulators of Metabotropic Glutamate Receptor 2

This highlight describes a series of novel 6-aryl isoindolin-1-one derivatives as negative allosteric modulators (NAMs) of the metabotropic glutamate receptor 2 (mGluR2). The chemical synthesis, selectivity over mGluR3, and pharmaceutical compositions of those compounds are disclosed.

CXC Chemokine Ligand 12 Facilitates Gi Protein Binding to CXC Chemokine Receptor 4 by Stabilizing Packing of the Proline-Isoleucine-Phenylalanine Motif: Insights from Automated Path Searching

The activation of G protein-coupled receptors (GPCRs) is a complex multibody multievent process involving agonist binding, receptor activation, G protein coupling, and subsequent G protein activation. The order and energetics of these events, though crucial for the rational design of selective GPCR drugs, are challenging to characterize and remain largely underexplored. Here, we employed molecular dynamics simulations and our recently developed traveling salesman-based automated path searching (TAPS) algorithm to efficiently locate the minimum free-energy paths for the coupling events of the CXC chemokine receptor 4 (CXCR4) with its endogenous ligand CXC chemokine ligand 12 (CXCL12) and Gi protein. We show that, after overcoming three low energy barriers (3.24-6.89 kcal/mol), Gi alone can precouple with CXCR4 even without CXCL12, consistent with previous reports on the existence of the apo CXCR4-Gi complex and our NanoBiT experiments. The highest barrier of 6.89 kcal/mol in this process corresponds to the packing of the proline-isoleucine-phenylalanine (PIF) motif of CXCR4. Interestingly, without Gi, CXCL12 alone cannot activate CXCR4 (high barrier of 18.89 kcal/mol). Instead, it can enhance Gi coupling by circumventing the energy barrier of PIF packing. Shedding new light on the activation mechanism of CXCR4, these results present TAPS as a promising tool for uncovering complete activation pathways of GPCRs and the corresponding agonist design.