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Abstract: Ordered protein-based assemblies are increasingly desirable for materials science, but the design of new materials remains challenging and requires considerable effort. Crystal lattice contact modulation enables rapid rational design of an assortment of structurally diverse constructs with crystalline order. Targeted disruption of crystal contacts and directional growth pre-biasing allows for restricting crystal lattice growth in selected directions, resulting in lower-dimension assemblies with parent crystal structural features. Two-dimensional crystals, one-dimensional fibres, flexible ribbons, and single-walled nanotubes based on tetratricopeptide repeat proteins were constructed from reengineered 3D crystal lattices. The large library of available crystal structures provides an abundance of engineering targets, promising to make crystal contact engineering a rapid and attractive approach for the design of ordered supramolecular protein assemblies.

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Abstract: Detergent solubilisation remains the most commonly used but potentially problematic method to extract membrane proteins from lipid bilayers for Cryo-EM studies. Although recent advances have introduced excellent alternatives—such as amphipols, nanodiscs and SMALPs—the use of detergents is often necessary for intermediate steps. In this paper, we share our experiences working with detergent-solubilised samples within the modern Cryo-EM structural pipeline from the perspective of an EM specialist. Our aim is to inform novice users about potential challenges they may encounter. Drawing on specific examples from a variety of biological membrane systems, including Magnesium channels, lipopolysaccharide biosynthesis, and the human major facilitator superfamily transporters, we describe how the intrinsic properties of detergent-extracted samples can affect protein purification, Cryo-EM grid preparation (including the formation of vitreous ice) and the reconstitution of proteins into micelles. We also discuss how these unique characteristics can impact different stages of structural analysis and lead to complications in single-particle averaging software analysis. For each case, we present our insights into the underlying causes and suggest possible mitigations or alternative approaches.

Open Access

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Abstract: Collagen VI links the cell surface to the extracellular matrix to provide mechanical strength to most mammalian tissues, and is linked to human diseases including muscular dystrophy, fibrosis, cardiovascular disease and osteoarthritis. Collagen VI assembles from heterotrimers of three different α-chains into microfibrils, but there are many gaps in our knowledge of the molecular assembly process. Here, we determine the structures of both heterotrimeric mini-collagen VI constructs and collagen VI microfibrils, from mammalian tissue, using cryogenic-electron microscopy. These structures reveal a cysteine-rich coiled coil region involved in trimerisation as well as microfibril assembly. Furthermore, our structures show that pathogenic mutations are located at interaction sites involved in different steps of collagen VI assembly, from the trimeric-coiled coil region that mediates heterotrimerisation, to clusters of mutations in the triple-helical region involved in microfibril formation. Our microfibril structure provides a template for understanding supramolecular assembly, and offers a platform for rationale design of therapeutics for collagen VI pathologies.

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Abstract: The extracellular space (apoplast) in plants is a key battleground during microbial infections. To avoid recognition, the bacterial model phytopathogen Pseudomonas syringae pv. tomato DC3000 produces glycosyrin. Glycosyrin inhibits the plant-secreted β-galactosidase BGAL1, which would otherwise initiate the release of immunogenic peptides from bacterial flagellin. Here, we report the structure, biosynthesis, and multifunctional roles of glycosyrin. High-resolution cryo–electron microscopy and chemical synthesis revealed that glycosyrin is an iminosugar with a five-membered pyrrolidine ring and a hydrated aldehyde that mimics monosaccharides. Glycosyrin biosynthesis was controlled by virulence regulators, and its production is common in bacteria and prevents flagellin recognition and alters the extracellular glycoproteome and metabolome of infected plants. These findings highlight a potentially wider role for glycobiology manipulation by plant pathogens across the plant kingdom.

Open Access

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Abstract: Bacterial RNA polymerase (RNAP) is a multisubunit enzyme that copies DNA into RNA in a process known as transcription. Bacteria use σ factors to recruit RNAP to promoter regions of genes that need to be transcribed, with 60% bacteria containing at least one specialized σ factor, σ54. σ54 recruits RNAP to promoters of genes associated with stress responses and forms a stable closed complex that does not spontaneously isomerize to the open state where promoter DNA is melted out and competent for transcription. The σ54-mediated open complex formation requires specific AAA+ proteins (ATPases Associated with diverse cellular Activities) known as bacterial enhancer-binding proteins (bEBPs). We have now obtained structures of new intermediate states of bEBP-bound complexes during transcription initiation, which elucidate the mechanism of DNA melting driven by ATPase activity of bEBPs and suggest a mechanistic model that couples the Adenosine triphosphate (ATP) hydrolysis cycle within the bEBP hexamer with σ54 unfolding. Our data reveal that bEBP forms a nonplanar hexamer with the hydrolysis-ready subunit located at the furthest/highest point of the spiral hexamer relative to the RNAP. ATP hydrolysis induces conformational changes in bEBP that drives a vectoral transiting of the regulatory N terminus of σ54 into the bEBP hexamer central pore causing the partial unfolding of σ54, while forming specific bEBP contacts with promoter DNA. Furthermore, our data suggest a mechanism of the bEBP AAA+ protein that is distinct from the hand-over-hand mechanism proposed for many other AAA+ proteins, highlighting the versatile mechanisms utilized by the large protein family.