Visualizing the native cellular organization by coupling cryofixation with expansion microscopy (Cryo-ExM)

Cryofixation has proven to be the gold standard for efficient preservation of native cell ultrastructure compared to chemical fixation, but this approach is not widely used in fluorescence microscopy owing to implementation challenges. Here, we develop Cryo-ExM, a method that preserves native cellular organization by coupling cryofixation with expansion microscopy. This method bypasses artifacts associated with chemical fixation and its simplicity will contribute to its widespread use in super-resolution microscopy.

Membrane-active Polymers: NCMNP13-x, NCMNP21-x and NCMNP21b-x for Membrane Protein Structural Biology

Membrane proteins are a ubiquitous group of bio-macromolecules responsible for many crucial biological processes and serve as drug targets for a wide range of modern drugs. Detergent-free technologies such as styrene-maleic acid lipid particles (SMALP), diisobutylene-maleic acid lipid particles (DIBMALP), and native cell membrane nanoparticles (NCMN) systems have recently emerged as revolutionary alternatives to the traditional detergent-based approaches for membrane protein research. NCMN systems aim to create a membrane-active polymer library suitable for high-resolution structure determination. Herein, we report our design, synthesis, characterization and comparative application analyses of three novel classes of NCMN polymers, NCMNP13-x, NCMNP21-x and NCMNP21b-x. Although each NCMN polymer can solubilize various model membrane proteins and conserve native lipids into NCMN particles, only the NCMNP21b-x series reveals lipid-protein particles with good buffer compatibility and high homogeneity suitable for single-particle cryo-EM analysis. Consequently, the NCMNP21b-x polymers that bring out high-quality NCMN particles are particularly attractive for membrane protein structural biology.

Metabolic Processing of Selenium-based Bioisostere of meso-diaminopimelic Acid in Live Bacteria

The bacterial cell wall supports cell shape and prevents lysis due to internal turgor pressure. A primary component of all known bacterial cell walls is the peptidoglycan (PG) layer, which is comprised of repeating units of sugars connected to short and unusual peptides. The various steps within PG biosynthesis are often the target of antibiotics as they are essential for cellular growth and survival. Synthetic mimics of PG have proven to be indispensable tools to study bacterial cell growth and remodeling. Yet, a common component of PG, meso-diaminopimelic acid (m-DAP) at the third position of the stem peptide, remains challenging to build synthetically and is not commercially available. Here, we describe the synthesis and metabolic processing of a selenium-based bioisostere of a m-DAP analogue, selenolanthionine. We show that selenolanthionine is installed within the PG of live bacteria by the native cell wall crosslinking machinery in several mycobacteria species. We envision that this probe will supplement the current methods available for investigating PG crosslinking in m-DAP containing organisms.

Cryo-EM snapshots of a native lysate provide structural insights into a metabolon-embedded transacetylase reaction

Found across all kingdoms of life, 2-keto acid dehydrogenase complexes possess prominent metabolic roles and form major regulatory sites. Although their component structures are known, their higher-order organization is highly heterogeneous, not only across species or tissues but also even within a single cell. Here, we report a cryo-EM structure of the fully active Chaetomium thermophilum pyruvate dehydrogenase complex (PDHc) core scaffold at 3.85 Å resolution (FSC = 0.143) from native cell extracts. By combining cryo-EM with macromolecular docking and molecular dynamics simulations, we resolve all PDHc core scaffold interfaces and dissect the residing transacetylase reaction. Electrostatics attract the lipoyl domain to the transacetylase active site and stabilize the coenzyme A, while apolar interactions position the lipoate in its binding cleft. Our results have direct implications on the structural determinants of the transacetylase reaction and the role of flexible regions in the context of the overall 10 MDa PDHc metabolon architecture.

Single-molecule imaging with cell-derived nanovesicles reveals early binding dynamics at a cyclic nucleotide-gated ion channel

Ligand binding to membrane proteins is critical for many biological signaling processes. However, individual binding events are rarely directly observed, and their asynchronous dynamics are occluded in ensemble-averaged measures. For membrane proteins, single-molecule approaches that resolve these dynamics are challenged by dysfunction in non-native lipid environments, lack of access to intracellular sites, and costly sample preparation. Here, we introduce an approach combining cell-derived nanovesicles, microfluidics, and single-molecule fluorescence colocalization microscopy to track individual binding events at a cyclic nucleotide-gated TAX-4 ion channel critical for sensory transduction. Our observations reveal dynamics of both nucleotide binding and a subsequent conformational change likely preceding pore opening. Kinetic modeling suggests that binding of the second ligand is either independent of the first ligand or exhibits up to ~10-fold positive binding cooperativity. This approach is broadly applicable to studies of binding dynamics for proteins with extracellular or intracellular domains in native cell membrane.

Cryo-EM Structure of Mechanosensitive Channel YnaI Using SMA2000: Challenges and Opportunities

Mechanosensitive channels respond to mechanical forces exerted on the cell membrane and play vital roles in regulating the chemical equilibrium within cells and their environment. High-resolution structural information is required to understand the gating mechanisms of mechanosensitive channels. Protein-lipid interactions are essential for the structural and functional integrity of mechanosensitive channels, but detergents cannot maintain the crucial native lipid environment for purified mechanosensitive channels. Recently, detergent-free systems have emerged as alternatives for membrane protein structural biology. This report shows that while membrane-active polymer, SMA2000, could retain some native cell membrane lipids on the transmembrane domain of the mechanosensitive-like YnaI channel, the complete structure of the transmembrane domain of YnaI was not resolved. This reveals a significant limitation of SMA2000 or similar membrane-active copolymers. This limitation may come from the heterogeneity of the polymers and nonspecific interactions between the polymers and the relatively large hydrophobic pockets within the transmembrane domain of YnaI. However, this limitation offers development opportunities for detergent-free technology for challenging membrane proteins.

The endoplasmic reticulum adopts two distinct tubule forms

The endoplasmic reticulum (ER) is a versatile organelle with diverse functions. Through super-resolution microscopy, we show that the peripheral ER in the mammalian cell adopts two distinct forms of tubules. Whereas an ultrathin form, R1, is consistently covered by ER-membrane curvature-promoting proteins, e.g., Rtn4 in the native cell, in the second form, R2, Rtn4 and analogs are arranged into two parallel lines at a conserved separation of ~105 nm over long ranges. The two tubule forms together account for ~90% of the total tubule length in the cell, with either one being dominant in different cell types. The R1-R2 dichotomy and the final tubule geometry are both co-regulated by Rtn4 (and analogs) and the ER sheet-maintaining protein Climp63, which respectively define the edge curvature and lumen height of the R2 tubules to generate a ribbon-like structure of well-defined width. Accordingly, the R2 tubule width correlates positively with the Climp63 intralumenal size. The R1 and R2 tubules undergo active remodeling at the second/sub-second time scales as they differently accommodate proteins, with the former effectively excluding ER-luminal proteins and ER-membrane proteins with large intraluminal domains. We thus uncover a dynamic structural dichotomy for ER tubules with intriguing functional implications.

Rotlicht-gesteuerter viraler Gentransfer mit Einzelzellauflösung

Available methods for efficient gene transfer into user-selected or even single cells suffer from high invasiveness or the need for complicated equipment. Here, we present a technology for the light-guided transduction of native cell lines and primary cells by adeno-associated viral (AAV) vectors. We demonstrate the spatially resolved transduction of different cells with different genes within one culture and the selective transduction of single cells by local illumination.

An Evolutionarily Conserved Regulatory Pathway of Muscle Mitochondrial Network Organization

Mitochondrial networks provide coordinated energy distribution throughout muscle cells. However, pathways specifying mitochondrial network-type separately from contractile fiber-type remain unclear. Here, we show that natural energetic demands placed on Drosophila melanogaster muscles yield native cell-types among which contractile and mitochondrial network-types are regulated independently. Proteomic analyses of indirect flight, jump, and leg muscles together with muscles misexpressing known fiber-type specification factor salm identified transcription factors H15 and cut as potential mitochondrial network regulators. We demonstrate H15 operates downstream of salm regulating flight muscle contractile and mitochondrial network-type. Conversely, H15 regulates mitochondrial network configuration but not contractile type in jump and leg muscles. Further, we find that cut regulates salm expression in flight muscles and mitochondrial network configuration in leg muscles. These data indicate cell type-specific regulation of muscle mitochondrial network organization separately from contractile type, mitochondrial content, and mitochondrial size through an evolutionarily conserved pathway involving cut, salm, and H15.