Biology subjects

Lea, S.

Publications and source records attributed to Lea, S..

8 recordsLinked to original sources

bioRxiv2026

Regulated conformational transitions in seipin define a functional ER–lipid droplet interface

Lipid droplets (LDs) are key organelles in cellular lipid homeostasis that form at the endoplasmic reticulum (ER) through a sequence of membrane rearrangements. While seipin emerged as an essential protein complex for LD biogenesis, how seipin-mediated LD formation proceeds beyond the initial step of neutral lipid nucleation remains unknown. Using a combination of in vitro and in-cell cryogenic electron microscopy (cryo-EM), ultrastructural expansion microscopy, molecular simulations and tailored genetic perturbations, we show that the seipin transmembrane domains undergo large-scale conformational rearrangements that define the architecture of the ER-LD interface and enable LD growth. Cryo-EM of purified Xenopus seipin revealed coexistence of two states: a compact "closed" conformation, consistent with early LD biogenesis, and an "open" conformation in which the transmembrane helices splay out laterally. Molecular dynamics simulations indicate that this open state induces local membrane curvature and promotes triacylglycerol accumulation. We identify conserved flexible linkers between the seipin luminal and transmembrane regions that act as mechanical hinges, enabling this conformational transition. We demonstrate that mutations in these hinge regions hinder seipin opening and affect LD formation in yeast and human cells. Analysis of native ER-LD contacts in human cells using light microscopy and cryo-electron tomography confirms that the seipin complex opens to establish stereotypical ~21-nm necks connecting the ER bilayer and LD monolayer. Moreover, we identify the liver-enriched microprotein SMLR1 as an inhibitor of this seipin conformational transition, providing a regulatory mechanism for seipin-dependent lipid storage in a tissue-specific manner. Together, these data establish seipin opening as a key structural rearrangement at the ER-LD interface that is essential for LD biogenesis and growth.

Cell Biology↗

bioRxiv2025

Identification and Characterization of a Small Molecule Ligand for the Huntingtin-HAP40 Complex

Huntingtons disease (HD) is caused by a CAG repeat expansion mutation, giving rise to a polyglutamine expansion in the huntingtin (HTT). However, the explicit molecular functions of HTT and opportunities for direct pharmacological modulation remain incompletely understood. Here, we report the discovery of a small molecule ligand for the full-length HTT protein in complex with its partner, HAP40. Using affinity selection mass spectrometry (AS-MS), we identified a stereoselective binder, whose binding was characterized by surface plasmon resonance, hydrogen-deuterium exchange mass spectrometry, and cryo-electron microscopy at 2.3 [A] resolution. The ligand binds HTT-HAP40 in vitro with single-digit micromolar affinity and one-to-one stoichiometry at a druggable interface previously predicted computationally. In silico studies predicted and experimental analyses confirmed the (R)-enantiomer as the eutomer and initial structure activity relationship was established experimentally. This work details a structurally-validated chemical scaffold and highlights a ligandable pocket which could enable development of chemical probes for probing HTT biology, as well as therapeutics such as degraders and imaging agents for HD.

pharmacology and toxicology↗

bioRxiv2020

The role of toxin:antitoxin systems and insertion sequences in the loss of virulence in Shigella sonnei

The Shigella plasmid, pINV, contains a 30 kb pathogenicity island (PAI) encoding a Type III secretion system (T3SS) which is essential for virulence. During growth in the laboratory, avirulent colonies of Shigella (which do not express a T3SS) arise spontaneously. Avirulence in Shigella flexneri mostly follows loss of the PAI, following recombination between insertion sequences (ISs) on pINV; toxin:antitoxin (TA) systems on pINV promote its retention through post-segregational killing (PSK). We show that avirulence in Shigella sonnei mainly results from plasmid loss, consistent with previous findings; IS-mediated PAI deletions can occur in S. sonnei, but through different ISs than in S. flexneri. We investigated the molecular basis for frequent loss of the S. sonnei plasmid, pINVSsonn. Introduction into pINVSsonn of CcdAB and GmvAT, toxin:antitoxin TA systems in pINV from S. flexneri but not S. sonnei, reduced plasmid loss and the emergence of avirulent bacteria. However, plasmid loss remained the leading cause of avirulence. We show that a single amino acid difference in the VapC toxin of the VapBC TA system in pINV also contributes to high frequency plasmid loss in S. sonnei compared to S. flexneri. Our findings demonstrate that the repertoire of ISs, complement of TA systems, and polymorphisms in TA systems influence plasmid dynamics and virulence loss in S. sonnei. Understanding the impact of polymorphisms should be informative about how TA systems contribute to PSK, and could be exploited for generating strains with stable plasmids.

microbiology↗

bioRxiv2019

Structure of the bacterial flagellar rotor MS-ring: a minimum inventory/maximum diversity system

The bacterial flagellum is a complex, self-assembling, nanomachine that confers motility on the cell. Despite great variation across species, all flagella are ultimately constructed from a helical propellor attached to a motor embedded in the inner membrane. The motor consists of a series of stator units surrounding a central rotor made up of two ring complexes, the MS-ring and the C-ring. Despite many studies, high resolution structural information is still completely lacking for the MS-ring of the rotor, and proposed mismatches in stoichiometry between the two rings have long provided a source of confusion for the field. We here present structures of the Salmonella MS-ring, revealing an unprecedented level of inter- and intra-chain symmetry variation that provides a structural explanation for the ability of the MS-ring to function as a complex and elegant interface between the two main functions of the flagellum, protein secretion and rotation.

microbiology↗

bioRxiv2019

Structural control for the coordinated assembly into functional pathogenic type-3 secretion systems

Functional injectisomes of the type-3 secretion system assemble into highly defined and stoichiometric bacterial molecular machines essential for infecting human and other eukaryotic cells. However, the mechanism that governs the regulated step-wise assembly process from the nucleation-phase, to ring-assembly, and the filamentous phase into a membrane embedded needle complex is unclear. We here report that the formation of a megadalton-sized needle complexes from Salmonella enterica serovar Typhimurium (SPI-1, Salmonella pathogenicity island-1) with proper stoichiometries is highly structurally controlled competing against the self-assembly propensity of injectisome components, leading to a highly unusual structurally-pleiotropic phenotype. The structure of the entire needle complex from pathogenic injectisomes was solved by cryo electron microscopy, focused refinements (2.5-4 [A]) and co-variation analysis revealing an overall asymmetric arrangement containing cyclic, helical, and asymmetric sub-structures. The centrally located export apparatus assembles into a conical, pseudo-helical structure and provides a structural template that guides the formation of a 24-mer cyclic, surrounding ring, which then serves as a docking interface comprising three different conformations for sixteen N-terminal InvG subunits of the outer secretin ring. Unexpectedly, the secretin ring excludes the 16th protein chain at the C-terminal outer ring, resulting in a pleiotropic 16/15-mer ring and consequently to an overall 24:16/15 basal body structure. Finally, we report how the transition from the pseudo-helical export apparatus into the helical filament is structurally resolved to generate the protein secretion channel, which provides the structural basis to restrict access of unfolded effector substrates. These results highlight the diverse molecular signatures required for a highly coordinated assembly process and provide the molecular basis for understanding triggering and transport of unfolded proteins through injectisomes.

molecular biology↗

bioRxiv2019

The structure of SALSA/DMBT1 SRCR domains reveal the conserved ligand-binding mechanism of the ancient SRCR-fold

The scavenger receptor cysteine-rich (SRCR) family of proteins comprise more than 20 membrane-associated and secreted molecules. Characterised by the presence of one or more copies of the ~110 amino acid SRCR domain, this class of proteins have widespread functions as anti-microbial molecules, scavenger- and signalling-receptors. Despite the high level of structural conservation of SRCR domains, no molecular basis for ligand interaction has been described. The SRCR protein SALSA, also known as dmbt1/gp340, is a key player in mucosal immunology. Based on detailed structures of the SALSA SRCR domains 1 and 8, we here reveal a novel universal ligand binding mechanism for SALSA ligands. The binding interface incorporates a dual cation binding site, which is highly conserved across the SRCR super family. Along with the well-described cation dependency on most SRCR domain-ligand interactions, our data suggest that the binding mechanism described for the SALSA SRCR domains is applicable to all SRCR domains. We thus propose to have identified in SALSA a conserved functional mechanism for ligand recognition by the SRCR class of proteins.

biochemistry↗

bioRxiv2019

The flagellar substrate specificity switch protein FlhB assembles onto the extra-membrane export gate to regulate type three secretion

Export of proteins through type three secretion systems (T3SS) is critical for motility and virulence of many major bacterial pathogens. Proteins are transported through an export gate complex consisting of three proteins (FliPQR in flagellar systems, SctRST in virulence systems) that were initially annotated as membrane proteins, but which we have recently shown assemble into an extra-membranous helical assembly. A fourth putative membrane protein (FlhB/SctU) is essential to the export process, and also functions to \"switch\" secretion substrate specificity once the growing hook/needle structures reach their determined length. Here we present the structure of an export gate containing the switch protein from a Vibrio polar flagellar system at 3.2 [A] resolution by cryo-electron microscopy. The structure reveals that the FlhB/SctU further extends the helical export gate assembly with its four putative transmembrane helices adopting an out-of-membrane location, wrapped around the other export gate components at the base of the structure. The unusual topology of the switch protein helices creates a loop that wraps around the bottom of the closed export gate complex. Structure-informed mutagenesis suggests that this loop is critical in gating secretion and we propose that a series of conformational changes in the type 3 secretion system trigger opening of the export gate through the interactions between FlhB/SctU and FliPQR/SctRST.

microbiology↗

bioRxiv2017

Structural and functional dissection of the interplay between lipid and Notch binding by human Notch ligands

AbstractRecent data have expanded our understanding of Notch signaling by identifying a C2 domain at the N-terminus of Notch ligands which has both lipid- and receptor-binding properties. We present novel structures of human ligands Jagged2 and DLL4 and human Notch-2, together with functional assays, which suggest that ligand-mediated coupling of membrane recognition and Notch binding is likely to be critical in establishing the optimal context for Notch signaling. Comparisons between the Jagged and Delta family show a huge diversity in the structures of the loops at the apex of the C2 domain implicated in membrane recognition and Jagged1 missense mutations which affect these loops and are associated with extrahepatic biliary atresia lead to a loss of membrane recognition, but do not alter Notch binding. Taken together, these data suggest that C2 domain binding to membranes is an important element in tuning ligand-dependent Notch signaling in different physiological contexts.

biochemistry↗