Summary The Melanocortin-4 Receptor (MC4R) plays a pivotal role in energy homeostasis. We used human MC4R mutations associated with an increased or decreased risk of obesity to dissect mechanisms that regulate MC4R function. Most obesity-associated mutations impair trafficking to the plasma membrane (PM), whereas obesity-protecting mutations either accelerate recycling to the PM or decrease internalization, resulting in enhanced signaling. MC4R mutations that do not affect canonical Gα s protein-mediated signaling, previously considered to be non-pathogenic, nonetheless disrupt agonist-induced internalization, β-arrestin recruitment, and/or coupling to Gα s , establishing their causal role in severe obesity. Structural mapping reveals ligand-accessible sites by which MC4R couples to effectors and residues involved in the homodimerization of MC4R, which is disrupted by multiple obesity-associated mutations. Human genetic studies reveal that endocytosis, intracellular trafficking, and homodimerization regulate MC4R function to a level that is physiologically relevant, supporting the development of chaperones, agonists, and allosteric modulators of MC4R for weight loss therapy.

The Duffy antigen receptor, also known as FY glycoprotein or CD234, is a seven transmembrane protein expressed primarily at the surface of red blood cells, which displays promiscuous binding to multiple chemokines. Not only does it serve as the basis of the Duffy blood group system but it also acts as the primary attachment site for malarial parasite Plasmodium vivax on erythrocytes and as one of the nucleating receptors for the pore forming toxins secreted by Staphylococcus aureus. Despite a predicted 7TM architecture and efficient binding to a spectrum of chemokines, it fails to exhibit canonical second messenger response such as calcium release, likely due to a lack of G protein coupling. Unlike prototypical GPCRs and β-arrestin-biased atypical chemokine receptors, the Duffy antigen receptor also appears to lack β-arrestin binding, making it an enigmatic 7TM chemokine receptor. In order to decipher the molecular mechanism of this intriguing functional divergence exhibited by the Duffy antigen receptor, we have determined its cryo-EM structure in complex with a C-C type chemokine, CCL7. The structure reveals a relatively superficial binding mode of CCL7, with the N-terminus of the receptor serving as the key interaction interface, and a partially formed orthosteric binding pocket lacking the second site for chemokine recognition compared to prototypical chemokine receptors. The structural framework allows us to employ HDX-MS approach to uncover ligand-induced structural changes in the receptor and draw important insights into the promiscuous nature of chemokine binding. Interestingly, we also observe a dramatic shortening of TM5 and 6 on the intracellular side, compared to prototypical GPCRs, which precludes the coupling of canonical signal-transducers namely G proteins, GRKs and β-arrestins, as demonstrated through extensive cellular assays. Taken together, our study uncovers a previously unknown structural mechanism that imparts unique functional divergence on the 7TM fold encoded in the Duffy antigen receptor while maintaining its scavenging function and should facilitate the designing of novel therapeutics targeting this receptor.

US28 is a viral G protein-coupled receptor (GPCR) encoded by the human cytomegalovirus (HCMV). This receptor, expressed both during lytic replication and viral latency, is required for latency. US28 is binding to a wide variety of chemokines but also exhibits a particularly high constitutive activity robustly modulating a wide network of cellular pathways altering the host cell environment to benefit HCMV infection. Several studies suggest that US28-mediated signalling may contribute to cancer progression. In this review, we discuss the unique structural characteristics that US28 acquired through evolution that confer a robust constitutive activity to this viral receptor. We also describe the wide downstream signalling network activated by this constitutive activation of US28 and discuss how these signalling pathways may promote and support important cellular aspects of cancer.

Chemokine receptors are GPCRs that regulate chemotactic migration of a wide variety of cells including immune and cancer cells. Most chemokine receptors contain features associated with the ability to stimulate G proteins during β-arrestin-mediated internalization into endosomes. As endosomal signaling of certain non-GPCR receptors plays a major role in cell migration, we chose to investigate the potential role of endosomal chemokine receptor signaling on mechanisms governing this function. Applying cell biological approaches and spatiotemporal-resolved proteome profiling, we demonstrate that the model chemokine receptor CCR7 recruits G protein and β-arrestin simultaneously upon chemokine stimulation enabling internalized receptors to activate G protein from endosomes. Furthermore, endosomal CCR7 uniquely enriches specific Rho-GTPase regulators as compared to plasma membrane CCR7, which is associated with enhanced activity of Rho-GTPase Rac1. As Rac1 drives the formation of membrane protrusions during chemotaxis, our findings suggest an important integrated function of endosomal chemokine receptor signaling in cell migration.

G protein-coupled receptors (GPCRs) can engage distinct subsets of signaling pathways, but the structural determinants of this functional selectivity remain elusive. The naturally occurring genetic variants of GPCRs, selectively affecting different pathways, offer an opportunity to explore this phenomenon. We previously identified 40 coding variants of the MTNR1B gene encoding the melatonin MT 2 receptor (MT 2 ). These mutations differently impact the β-arrestin 2 recruitment, ERK activation, cAMP production, and Gα i1 and Gα z activation. In this study, we combined functional clustering and structural modeling to delineate the molecular features controlling the MT 2 functional selectivity. Using non-negative matrix factorization, we analyzed the signaling signatures of the 40 MT 2 variants yielding eight clusters defined by unique signaling features and localized in distinct domains of MT 2 . Using computational homology modeling, we describe how specific mutations can selectively affect the subsets of signaling pathways and offer a proof of principle that natural variants can be used to explore and understand the GPCR functional selectivity.