Proteins are the primary functional actors of the cell. While proteoform diversity is known to be highly biologically relevant, current protein analysis methods are of limited use for distinguishing proteoforms. Mass spectrometric methods, in particular, often provide only ambiguous information on post-translational modification sites, and sequences of co-existing modifications may not be resolved. Here we demonstrate fluorescence resonance energy transfer (FRET)-based single-molecule protein fingerprinting to map the location of individual amino acids and post-translational modifications within single full-length protein molecules. Our data show that both intrinsically disordered proteins and folded globular proteins can be fingerprinted with a subnanometer resolution, achieved by probing the amino acids one by one using single-molecule FRET via DNA exchange. This capability was demonstrated through the analysis of alpha-synuclein, an intrinsically disordered protein, by accurately quantifying isoforms in mixtures using a machine learning classifier, and by determining the locations of two O-GlcNAc moieties. Furthermore, we demonstrate fingerprinting of the globular proteins Bcl-2-like protein 1, procalcitonin and S100A9. We anticipate that our ability to perform proteoform identification with the ultimate sensitivity may unlock exciting new venues in proteomics research and biomarker-based diagnosis@en

Guanylate-binding proteins (GBPs) are interferon-inducible guanosine triphosphate hydrolases (GTPases) mediating host defense against intracellular pathogens. Their antimicrobial activity hinges on their ability to self-associate and coat pathogen-associated compartments or cytosolic bacteria. Coat formation depends on GTPase activity but how nucleotide binding and hydrolysis prime coat formation remains unclear. Here, we report the cryo-electron microscopy structure of the full-length human GBP1 dimer in its guanine nucleotide-bound state and describe the molecular ultrastructure of the GBP1 coat on liposomes and bacterial lipopolysaccharide membranes. Conformational changes of the middle and GTPase effector domains expose the isoprenylated C terminus for membrane association. The α-helical middle domains form a parallel, crossover arrangement essential for coat formation and position the extended effector domain for intercalation into the lipopolysaccharide layer of gram-negative membranes. Nucleotide binding and hydrolysis create oligomeric scaffolds with contractile abilities that promote membrane extrusion and fragmentation. Our data offer a structural and mechanistic framework for understanding GBP1 effector functions in intracellular immunity.@en

Cryogenic electron tomography (cryo-ET) combined with sub-tomogram averaging, allows in-situ visualization and structure determination of macromolecular complexes at sub-nanometre resolution. Cryogenic focused ion beam (cryo-FIB) micromachining is used to prepare a thin lamella-shaped sample out of a frozen-hydrated cell for cryo-ET imaging, but standard cryo-FIB fabrication is blind to the precise location of the structure or proteins of interest. Fluorescence-guided focused ion beam (FIB) milling at target locations requires multiple sample transfers prone to contamination, and relocation and registration accuracy is often insufficient for 3D targeting. Here, we present in-situ fluorescence microscopy-guided FIB fabrication of a frozen-hydrated lamella to address this problem: we built a coincident 3-beam cryogenic correlative microscope by retrofitting a compact cryogenic microcooler, custom positioning stage, and an inverted widefield fluorescence microscope (FM) on an existing focused ion-beam scanning electron microscope (FIB-SEM). We show FM controlled targeting at every milling step in the lamella fabrication process, validated with transmission electron microscope (TEM) tomogram reconstructions of the target regions. The ability to check the lamella during and after the milling process results in a higher success rate in the fabrication process and will increase the throughput of fabrication for lamellae suitable for high-resolution imaging.

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Membrane proteins are known to exert many essential biological functions by forming complexes in cell membranes. An example refers to the β-barrel assembly machinery (BAM), a 200 kDa pentameric complex containing BAM proteins A–E that catalyzes the essential process of protein insertion into the outer membrane of gram-negative bacteria. While progress has been made in capturing three-dimensional structural snapshots of the BAM complex, the role of the lipoprotein BamC in the complex assembly in functional lipid bilayers has remained unclear. We have devised a component-selective preparation scheme to directly study BamC as part of the entire BAM complex in lipid bilayers. Combination with proton-detected solid-state NMR methods allowed us to probe the structure, dynamics, and supramolecular topology of full-length BamC embedded in the entire complex in lipid bilayers. Our approach may help decipher how individual proteins contribute to the dynamic formation and functioning of membrane protein complexes in membranes.

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For a long time, solid-state nuclear magnetic resonance (ssNMR) has been employed to study complex biomolecular systems at the detailed chemical, structural, or dynamic level. Recent progress in high-resolution and high-sensitivity ssNMR, in combination with innovative sample preparation and labeling schemes, offers novel opportunities to study proteins in their native setting irrespective of the molecular tumbling rate. This protocol describes biochemical preparation schemes to obtain cellular samples of both soluble as well as insoluble or membrane-associated proteins in bacteria. To this end, the protocol is suitable for studying a protein of interest in both whole cells and in cell envelope or isolated membrane preparations. In the first stage of the procedure, an appropriate strain of Escherichia coli (DE3) is transformed with a plasmid of interest harboring the protein of interest under the control of an inducible T7 promoter. Next, the cells are adapted to grow in minimal (M9) medium. Before the growth enters stationary phase, protein expression is induced, and shortly thereafter, the native E. coli RNA polymerase is inhibited using rifampicin for targeted labeling of the protein of interest. The cells are harvested after expression and prepared for ssNMR rotor filling. In addition to conventional ¹³C/¹⁵N-detected ssNMR, we also outline how these preparations can be readily subjected to multidimensional ssNMR experiments using dynamic nuclear polarization (DNP) or proton (¹H) detection schemes. We estimate that the entire preparative procedure until NMR experiments can be started takes 3–5 days.

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