Cyclic nucleotide-gated (CNG) ion channels, despite a significant homology with the highly selective K + channels, do not discriminate among monovalent alkali cations and are permeable also to several organic cations. We combined electrophysiology, molecular dynamics (MD) simulations, and X-ray crystallography to demonstrate that the pore of CNG channels is highly flexible. When a CNG mimic is crystallized in the presence of a variety of monovalent cations, including Na + , Cs + , and dimethylammonium (DMA + ), the side chain of Glu66 in the selectivity filter shows multiple conformations and the diameter of the pore changes significantly. MD simulations indicate that Glu66 and the prolines in the outer vestibule undergo large fluctuations, which are modulated by the ionic species and the voltage. This flexibility underlies the coupling between gating and permeation and the poor ionic selectivity of CNG channels.CNG channels | pore flexibility | X-ray crystallography | MD simulations I n K + selective channels, the opening and closing of the ion channel pore (gating) and the translocation of ions through the pore (permeation) are considered independent processes with distinct structural basis (1). Gating is controlled by the bundle crossing at the intracellular side, whereas permeation reflects ion-ion and ion-pore interactions within the selectivity filter (1-5). Based on these experimental observations, the current paradigm assumes that the 3D structure of the selectivity filter is relatively rigid during ion translocation and the mechanisms of ionic permeation can be deduced in essence from its crystal structure. This paradigm has been successfully applied to several K + channels (4, 6, 7). Cyclic nucleotide-gated (CNG) channels underlie sensory transduction in the retina and olfactory epithelium and share a high degree of homology with K + channels (8-10). In contrast to K + channels, CNG channels' primary gate is located at the selectivity filter (11), suggesting that the same protein region controls ion permeation and gating. In CNG channels the ionic species present inside the pore influences channel gating; however, the nature of this coupling is not well understood (12-16). In the presence of large cations, such as Rb + and Cs + , channel conductance and gating are also controlled by membrane voltage, and current-voltage relationships activated by 1 mM cGMP depend on the radius of the permeating ion (17, 18) (Fig. S1).Structural information on CNG channels is limited to a lowresolution electron microscopy map (19), partial crystal structures of the intracellular cyclic nucleotide-binding domains (20)(21)(22), and the crystal structure of a chimeric channel in which the CNG selectivity-filter sequence is engineered into a bacterial NaK channel, creating a CNG mimic (NaK2CNG; Fig. 1 A and B) that shares several properties of CNG channels (23). This CNG mimic provides a suitable model for understanding the properties of the pore underlying the low ionic selectivity and the coupling between gating and perme...

Mutations in the STK11 (LKB1) gene regulate resistance to PD-1/PD-L1 blockade. This study evaluated this association in patients with nonsquamous non-small-cell lung cancer enrolled in three Phase 1/2 trials. STK11 mutations were associated with resistance to the anti-PD-L1 antibody durvalumab (alone/with the anti-CTLA-4 antibody tremelimumab) independently of KRAS mutational status, highlighting STK11 as a potential driver of resistance to checkpoint blockade. Retrospective assessments of tumor tissue, whole blood and serum revealed a unique immune phenotype in patients with STK11 mutations, with increased expression of markers associated with neutrophils (i.e. CXCL2, IL6), Th17 contexture (i.e. IL17A) and immune checkpoints. Associated changes were observed in the periphery. Reduction of STAT3 in the tumor microenvironment using an antisense oligonucleotide reversed immunotherapy resistance in preclinical STK11 knockout models. These results suggest that STK11 mutations may hinder response to checkpoint blockade through mechanisms including suppressive myeloid cell biology, which could be reversed by STAT3-targeted therapy. SIGNIFICANCEPatients with nonsquamous STK11mut NSCLC are less likely than STK11wt patients to respond to anti-PD-L1 ± anti-CTLA-4 immunotherapies, and their tumors show increased expression of genes and cytokines that activate STAT3 signaling. Preclinically, STAT3 modulation reverses this resistance, suggesting STAT3-targeted agents as potential combination partners for immunotherapies in STK11mut NSCLC.

The recent Vibrio parahaemolyticus sodium/galactose (vSGLT) symporter crystal structure captures the protein in an inward-facing substrate-bound conformation, with the sodium ion placed, by structural alignment, in a site equivalent to the Na2 site of the leucine transporter (LeuT). A recent study, based on molecular dynamics simulations, showed that the sodium ion spontaneously leaves its initial position diffusing outside vSGLT, toward the intracellular space. This suggested that the crystal structure corresponds to an ion-releasing state of the transporter. Here, using metadynamics, we identified a more stable Na(+) binding site corresponding to a putative ion-retaining state of the transporter. In addition, our simulations, consistently with mutagenesis studies, highlight the importance of D189 that, without being one of the Na(+)-coordinating residues, regulates its binding/release.

Sodium-Galactose Transporter (SGLT) is a secondary active symporter which accumulates sugars into cells by using the electrochemical gradient of Na+ across the membrane. Previous computational studies provided insights into the release process of the two ligands (galactose and sodium ion) into the cytoplasm from the inward-facing conformation of Vibrio parahaemolyticus sodium/galactose transporter (vSGLT). Several aspects of the transport mechanism of this symporter remain to be clarified: (i) a detailed kinetic and thermodynamic characterization of the exit path of the two ligands is still lacking; (ii) contradictory conclusions have been drawn concerning the gating role of Y263; (iii) the role of Na+ in modulating the release path of galactose is not clear. In this work, we use bias-exchange metadynamics simulations to characterize the free energy profile of the galactose and Na+ release processes toward the intracellular side. Surprisingly, we find that the exit of Na+ and galactose is non-concerted as the cooperativity between the two ligands is associated to a transition that is not rate limiting. The dissociation barriers are of the order of 11–12 kcal/mol for both the ion and the substrate, in line with kinetic information concerning this type of transporters. On the basis of these results we propose a branched six-state alternating access mechanism, which may be shared also by other members of the LeuT-fold transporters.

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