Sigrid Noreng

C-X-C motif chemokine receptor 3 (CXCR3) is essential for immune cell functions and pivotal in T helper 1 cell infiltration in autoimmune and chronic inflammatory diseases and in tumor proliferation and metastasis, but the mechanisms by which the endogenous ligands CXCL9, CXCL10, and CXCL11 differentially recognize and activate CXCR3 are not fully understood. Here, we present cryo–electron microscopy structures of all three chemokine-CXCR3-G _i complexes, complemented by cell binding studies and functional mutagenesis data. We systematically compare the pharmacological and interaction profiles of CXCL9, CXCL10, and CXCL11 to rationalize their varying efficacies and potencies and to reveal the critical role of the membrane-distal CXCR3 N terminus in ligand binding and signaling. Using chimeric chemokines and molecular dynamics, we reveal the signaling plasticity of chemokine ligands and signaling determinants. Together, these insights enable us to propose a multimodal binding and activation framework that explains CXCR3 chemokine ligand multispecificity and signaling versatility and offer tools to interrogate and modulate CXCR3 biology.

NOX2 is the prototypical member of the NADPH oxidase NOX superfamily and produces superoxide (O₂^•−), a key reactive oxygen species (ROS) that is essential in innate and adaptive immunity. Mutations that lead to deficiency in NOX2 activity correlate with increased susceptibility to bacterial and fungal infections, resulting in chronic granulomatous disease. The core of NOX2 is formed by a heterodimeric transmembrane complex composed of NOX2 (formerly gp91) and p22, but a detailed description of its structural architecture is lacking. Here, we present the structure of the human NOX2 core complex bound to a selective anti-NOX2 antibody fragment. The core complex reveals an intricate extracellular topology of NOX2, a four-transmembrane fold of the p22 subunit, and an extensive transmembrane interface which provides insights into NOX2 assembly and activation. Functional assays uncover an inhibitory activity of the 7G5 antibody mediated by internalization-dependent and internalization-independent mechanisms. Overall, our results provide insights into the NOX2 core complex architecture, disease-causing mutations, and potential avenues for selective NOX2 pharmacological modulation.

The molecular bases of heteromeric assembly and link between Na⁺ self-inhibition and protease-sensitivity in epithelial sodium channels (ENaCs) are not fully understood. Previously, we demonstrated that ENaC subunits – α, β, and γ – assemble in a counterclockwise configuration when viewed from outside the cell with the protease-sensitive GRIP domains in the periphery (Noreng et al., 2018). Here we describe the structure of ENaC resolved by cryo-electron microscopy at 3 Å. We find that a combination of precise domain arrangement and complementary hydrogen bonding network defines the subunit arrangement. Furthermore, we determined that the α subunit has a primary functional module consisting of the finger and GRIP domains. The module is bifurcated by the α2 helix dividing two distinct regulatory sites: Na⁺ and the inhibitory peptide. Removal of the inhibitory peptide perturbs the Na⁺ site via the α2 helix highlighting the critical role of the α2 helix in regulating ENaC function.

The epithelial sodium channel (ENaC), a member of the ENaC/DEG superfamily, regulates Na⁺ and water homeostasis. ENaCs assemble as heterotrimeric channels that harbor protease-sensitive domains critical for gating the channel. Here, we present the structure of human ENaC in the uncleaved state determined by single-particle cryo-electron microscopy. The ion channel is composed of a large extracellular domain and a narrow transmembrane domain. The structure reveals that ENaC assembles with a 1:1:1 stoichiometry of α:β:γ subunits arranged in a counter-clockwise manner. The shape of each subunit is reminiscent of a hand with key gating domains of a ‘finger’ and a ‘thumb.’ Wedged between these domains is the elusive protease-sensitive inhibitory domain poised to regulate conformational changes of the ‘finger’ and ‘thumb’; thus, the structure provides the first view of the architecture of inhibition of ENaC.