Taisne, C. | bioRxiv Science

bioRxiv2024

Thickness and quality controlled fabrication of fluorescence-targeted frozen-hydrated lamellae

Cryogenic focused ion beam (FIB) milling is essential for fabricating thin lamella-shaped samples out of frozen-hydrated cells for high-resolution structure determination. Structural information can only be resolved at high resolution if the lamella thickness is between 100 and 200 nm. While the lamella fabrication workflow has undergone significant improvements since its conception, quantitative, live feedback on lamella thickness and quality is still lacking. Taking advantage of a coincident light microscopy integrated into the FIB-SEM, we present three different strategies that together allow accurate, live control during lamella fabrication. First, we combine 4D-STEM with fluorescence microscope (FM) targeting to determine the lamella thickness. Second, with reflected light microscopy (RLM) we screen target sites for ice contamination and monitor lamella thickness and integrity of the protective Pt coating during FIB milling. Third, we exploit thin-film interference to obtain fine-grained feedback on thickness uniformity below 500 nm. We finally present a full workflow for fluorescence-targeted and quality controlled fabrication of frozen-hydrated lamellae, benchmarked with excellent agreement to energy filtered transmision electron microscopy (EFTEM) measurements and reconstructed tomograms obtained with electron cryo-tomography.

biophysics↗

bioRxiv2023

Structural basis of membrane targeting and coatomer assembly by human GBP1

Guanylate-Binding Proteins (GBPs) are interferon-inducible guanosine triphosphate hydrolases (GTPases) that mediate immune effector functions against intracellular pathogens. A key step for the antimicrobial activity of GBPs is the formation of homo- and heterooligomeric complexes on the membrane of pathogen-associated compartments or cytosolinvasive bacteria. Similar to other large GTPases of the dynamin family, oligomerisation and membrane association of GBPs depend on their GTPase activity. How nucleotide binding and hydrolysis prime GBPs for membrane targeting and coatomer formation remains unclear. Here, we report the cryo-EM structure of the full-length human GBP1 dimer in its guanine nucleotide-bound state and resolve the molecular ultrastructure of GBP1 coatomer assemblies on liposomes and bacterial lipopolysaccharide membranes. We show how nucleotide binding promotes large-scale conformational changes of the middle and GTPase effector domains that expose the isoprenylated carboxyl-terminus for association with lipid membranes. Our structure reveals how the -helical stalks of the middle domain form a parallel arrangement firmly held in a unique cross-over arrangement by intermolecular contacts between adjacent monomers. This conformation is critical for GBP1 dimers to assemble into densely packed coatomers on target membranes. The extended -helix of the effector domain is flexible and permits intercalation into the dense lipopolysaccharide layer on the outer membrane of gram-negative bacterial pathogens. We show that nucleotide-dependent oligomerisation and GTP hydrolysis yield GBP1 membrane scaffolds with contractile abilities that promote the formation of tubular membrane protrusions and membrane fragmentation. Collectively, our data provide a structural and mechanistic framework for interrogating the molecular basis for GBP1 effector functions in intracellular immunity.

biophysics↗