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The C-terminal region of Clostridium perfringens enterotoxin (C-CPE) can bind to specific claudins, resulting in the disintegration of tight junctions (TJs) and an increase in the paracellular permeability across epithelial cell sheets. Here we present the structure of mammalian claudin-19 in complex with C-CPE at 3.7 Å resolution. The structure shows that C-CPE forms extensive hydrophobic and hydrophilic interactions with the two extracellular segments of claudin-19. The claudin-19/C-CPE complex shows no density of a short extracellular helix that is critical for claudins to assemble into TJ strands. The helix displacement may thus underlie C-CPE-mediated disassembly of TJs.

The gastric proton pump-the H, K-ATPase-is a P-type ATPase responsible for acidifying the gastric juice down to pH 1. This corresponds to a million-fold proton gradient across the membrane of the parietal cell, the steepest known cation gradient of any mammalian tissue. The H, K-ATPase is an important target for drugs that treat gastric acid-related diseases. Here we present crystal structures of the H, K-ATPase in complex with two blockers, vonoprazan and SCH28080, in the luminal-open state, at 2.8 Å resolution. The drugs have partially overlapping but clearly distinct binding modes in the middle of a conduit running from the gastric lumen to the cation-binding site. The crystal structures suggest that the tight configuration at the cation-binding site lowers the pK value of Glu820 sufficiently to enable the release of a proton even into the pH 1 environment of the stomach.

Tight junction is a cell adhesion apparatus functioning as barrier and/or channel in the paracellular spaces of epithelia. Claudin is the major component of tight junction and polymerizes to form tight junction strands with various morphologies that may correlate with their functions. Here we present the crystal structure of mammalian claudin-3 at 3.6 Å resolution. The third transmembrane helix of claudin-3 is clearly bent compared with that of other subtypes. Structural analysis of additional two mutants with a single mutation representing other subtypes in the third helix indicates that this helix takes a bent or straight structure depending on the residue. The presence or absence of the helix bending changes the positions of residues related to claudin-claudin interactions and affects the morphology and adhesiveness of the tight junction strands. These results evoke a model for tight junction strand formation with different morphologies – straight or curvy strands – observed in native epithelia.

Prokaryotic voltage-gated sodium channels (Na V s) are homotetramers and are thought to inactivate through a single mechanism, named C-type inactivation. Here we report the voltage dependence and inactivation rate of the NaChBac channel from Bacillus halodurans, the first identified prokaryotic Na V , as well as of three new homologues cloned from Bacillus licheniformis (Na V BacL), Shewanella putrefaciens (Na V SheP), and Roseobacter denitrificans (Na V RosD). We found that, although activated by a lower membrane potential, Na V BacL inactivates as slowly as NaChBac. Na V SheP and Na V RosD inactivate faster than NaChBac. Mutational analysis of helix S6 showed that residues corresponding to the "glycine hinge" and "PXP motif" in voltage-gated potassium channels are not obligatory for channel gating in these prokaryotic Na V s, but mutations in the regions changed the inactivation rates. Mutation of the region corresponding to the glycine hinge in Na V BacL (A214G), Na V SheP (A216G), and NaChBac (G219A) accelerated inactivation in these channels, whereas mutation of glycine to alanine in the lower part of helix S6 in NaChBac (G229A), Na V BacL (G224A), and Na V RosD (G217A) reduced the inactivation rate. These results imply that activation gating in prokaryotic Na V s does not require gating motifs and that the residues of helix S6 affect C-type inactivation rates in these channels.Voltage-gated sodium channels (Na V s) 3 generate the rapid upstroke of action potentials in nerve cell axons (1). In mammalian Na V s, the channel is formed by the ␣-subunit, which comprises four repeats of six-transmembrane segments, with each repeat consisting of 300 -400 amino acids. The ␣-subunit carries several glycosylation sites and co-assembles with auxiliary subunits to form the native channel (2, 3). The only structural information on Na V s available to date is a density map of the Na V from the electric organ of the electric eel determined by cryoelectron microscopy (4). Due to its limited resolution of 19 Å, the density map did not provide insights into the gating or sodium selectivity.The first prokaryotic Na V , NaChBac, was cloned from Bacillus halodurans (5). Subsequently, three more prokaryotic sodium channels were cloned and characterized (6, 7). All studied prokaryotic Na V s form homotetramers with a structure thought to be similar to that of some potassium channels with known structures (8 -10). Furthermore, because the proteins could be expressed in large amounts in Escherichia coli and purified by metal chelate affinity chromatography (5, 7, 11), they are promising candidates for high resolution structure determination and structure-function analyses.The physiological role of prokaryotic Na V s may be related to pH homeostasis, motility, and chemotaxis (6, 12). Searching bacterial genomic data bases, we found 26 sequences of putative NaChBac homologues from bacteria living in various environments. We were able to clone the putative Na V genes from three of these bacteria, Bacillus licheniformis, Shewanella pu...