Cryo-EM structures of Clostridium perfringens enterotoxin bound to its human receptor, claudin-4

Generation of Conformation-Specific Monoclonal Antibodies for Integral Membrane Proteins

Antibodies and their antigen-binding fragments, including fragment antigen-binding domains (Fabs) and single-chain variable fragments (scFvs), are extraordinary tools in all fields of biology, particularly in neuroscience, where they have been utilized for imaging, detection, and quantification studies. Most antibodies bind to unstructured or linear epitopes. Conformation-specific antibodies, by contrast, bind to 3D epitopes, recognizing native conformations of the target antigen, and have proven highly useful in X-ray crystallography as crystallization chaperones and in cryo-electron microscopy as fiducial markers. Moreover, because conformation-specific antibodies recognize 3D shapes of the antigen, they often have exquisite specificity and are useful in immunofluorescence studies and in isolation of antigen from native tissues. Over the past decade, our group has devoted effort to developing murine monoclonal antibodies (mAbs) against important synaptic receptors, particularly ionotropic glutamate receptors (iGluRs) and their auxiliary proteins. We have developed reproducible methods for generating high-quality mAbs for structural, biochemical, and imaging studies. In this article, we show how to prepare proteoliposomes (PLs), carry out immunization and track the immune response, perform hybridoma generation, and analyze the specificity, cross-reactivity, and competition of mAb binding via enzyme-linked immunosorbent assay and fluorescence-detection size-exclusion chromatography. Our PL-based method produces high-affinity, conformation-specific antibodies targeting diverse synaptic membrane receptors in 4 months. Here, we describe the relevant protocols in detail and document the mAbs, Fabs, and scFvs that we have produced against iGluRs and their auxiliary subunits. © 2025 The Author(s). Current Protocols published by Wiley Periodicals LLC. Basic Protocol 1: Generation of conformation-specific antibodies for integral membrane proteins Support Protocol 1: Detection of conformational antibodies using ELISA Basic Protocol 2: Expression and purification of monoclonal antibodies and their derivatives Support Protocol 2: Concentration and clarification of insect cell supernatant for Fab purification Support Protocol 3: Measurement of ionotropic glutamate receptor binding kinetics using Octet BLI System.

Trans-Mediated, Cis-Inhibited Paradoxal Activity of Clostridium perfringens Enterotoxin (c-CPE) in Modulating Epithelial Permeability

In the context of transdermal delivery, favoring the drug permeability of epithelia through convenient formulations would open new opportunities for local versus systemic drug delivery, envisaging higher patient comfort and an enhanced therapeutic effect. Ligands of tight junctions are interesting agents that enhance epithelial permeability by relaxing the protein complexes that form them. The C-terminal domain of Clostridium perfringens enterotoxin (c-CPE), which binds claudins, one of the tight junction (TJ) components, has been explored here as a functional domain in modular recombinant proteins, to evaluate its ability to self-promote its paracellular epithelial passage in a Caco-2 cell monolayer model. c-CPE-containing fusion proteins bind cells in the absence of internalization and cytotoxicity and support the passage, in trans, of other fusion proteins devoid of c-CPE. However, c-CPE-carrying proteins fail to cross the epithelia by themselves, probably because their affinity for TJs immobilizes them in the intercellular space. Therefore, while recombinant c-CPE versions have been here confirmed as convenient epithelial-permeabilizing agents, a paradoxical behavior has been observed where this effect is only successful when applied in trans, specifically on entities that lack c-CPE. Then, c-CPE itself inhibits the paracellular mobility of carrier molecules, not being suited as a self-driver (in c-CPE-drug complexes) for drug delivery through epithelia.

Structure of the lens MP20 mediated adhesive junction

Human lens fiber membrane intrinsic protein MP20 is the second most abundant membrane protein of the human eye lens. Despite decades of effort its structure and function remained elusive. Here, we determined the MicroED structure of full-length human MP20 in lipidic-cubic phase to a resolution of 3.5 Å. MP20 forms tetramers each of which contain 4 transmembrane α-helices that are packed against one another forming a helical bundle. We find that each MP20 tetramer formed adhesive interactions with an opposing tetramer in a head-to-head fashion. Investigation of MP20 localization in human lenses indicate that in young fiber cells MP20 is initially localized to the cytoplasm in differentiating fiber cells but upon fiber cell maturation is inserted into the plasma membrane, correlating with the restriction of the diffusion of extracellular tracers into the lens. Together these results suggest that MP20 forms lens thin junctions in vivo, confirming its role as a structural protein in the human eye lens essential for its optical transparency.

C. perfringens enterotoxin-claudin pore complex: Models for structure, mechanism of pore assembly and cation permeability

The pore-forming Clostridium perfringens enterotoxin (CPE), a common cause of foodborne diseases, facilitates Ca2+ influx in enterocytes, leading to cell damage. Upon binding to certain claudins (e.g., claudin-4), CPE forms oligomeric pores in the cell membrane. While the mechanism of CPE-claudin interaction is well understood, the structure and assembly of the pore complex remain elusive. Here, we used AlphaFold2 complex prediction, structure alignment, and molecular dynamics simulations to generate models of prepore and pore states of the CPE/claudin-4 complex. We sequentially addressed CPE-claudin, CPE-CPE, and claudin-claudin interactions, along with CPE conformational changes. The CPE pore is a hexameric variant of the typical heptameric pore stem and cap architecture of aerolysin-like β-barrel pore-forming toxins (β-PFT). The pore is lined with three hexa-glutamate rings, which differ from other β-PFTs and confer CPE-specific cation selectivity. Additionally, the pore center is indicated to be anchored by a dodecameric claudin ring formed by a cis-interaction variant of an interface found in claudin-based tight junction strands. Mutation of an interface residue inhibited CPE-mediated cell damage in vitro. We propose that this claudin ring constitutes an anchor for a twisting mechanism that drives extension and membrane insertion of the CPE β-hairpins. Our pore model agrees with previous key experimental data and provides insights into the structural mechanisms of CPE-mediated cytotoxic cation influx.