Uropathogenic Escherichia coli strains use filamentous type 1 pili to adhere to and invade uroepithelial cells. The pilus consists of a flexible tip fibrillum, formed by the adhesin FimH and the subunits FimG and FimF. The pilus rod is a helical assembly of up to 3000 copies of the main subunit FimA, terminated by a single copy of the subunit FimI that anchors the rod to the assembly platform FimD in the outer membrane. Although type 1 pilus assembly can be completely reconstituted in vitro, the precise mechanism of assembly termination on FimD is still unknown. Here, we present cryo-electron microscopy structures of the fully assembled pilus with all its components prior to and after incorporation of FimI, capped with the assembly chaperone FimC. The structures reveal that FimD positions the proximal end of the pilus rod at an angle of ca. 50 degrees relative to the plane of the outer membrane. Specific interactions between FimI and FimC, absent in the equivalent FimA-FimC interface of the non-terminated pilus, stabilize the assembly-terminated state. In addition, we present structures of the transition region between the tip fibrillum and the helical rod, showing how FimF aligns the tip fibrillum along the rod axis.

The DNA damage response in mycobacteria is controlled by the heterodimeric transcription factor PafBC, a member of the WYL domain-containing protein family. It has been shown that PafBC induces transcription of its regulon by reprogramming the housekeeping RNA polymerase holoenzyme to recognize PafBC-dependent promoters through sigma adaptation. However, the mechanism by which DNA damage is sensed and translated into PafBC activation has remained unclear. Here, we demonstrate that the binding of single-stranded DNA (ssDNA) to the WYL domains of PafBC activates the transcription factor. Our cryo-electron microscopy structure of full-length PafBC in its active conformation, bound to the transcription initiation complex, reveals a previously unknown mode of interaction between the ssDNA and the WYL domains. Using biochemical experiments, we show that short ssDNA fragments bind to PafBC dynamically, resulting in deactivation as ssDNA levels decrease postrepair. Our findings shed light on the mechanism linking DNA damage to PafBC activation and expand our understanding of WYL domain-containing proteins.

Mycobacteria harbor a proteasome that was acquired by Actinobacteria through horizontal gene transfer and that supports the persistence of the human pathogen Mycobacterium tuberculosis within host macrophages. The core particle of the proteasome (20S CP) associates with ring-shaped activator complexes to degrade protein substrates. One of these is the bacterial proteasome activator Bpa that stimulates the ATP-independent proteasomal degradation of the heat shock repressor HspR. In this study, we determine the cryogenic electron microscopy 3D reconstruction of the complex between Bpa and its natural substrate HspR at 4.1 Å global resolution. The resulting maps allow us to identify regions of Bpa that interact with HspR. Using structure-guided site-directed mutagenesis and in vitro biochemical assays, we confirm the importance of the identified residues for Bpa-mediated substrate recruitment and subsequent proteasomal degradation. Additionally, we show that the dodecameric Bpa ring associates asymmetrically with the heptameric α-rings of the 20S CP, adopting a conformation resembling a hinged lid, while still engaging all seven docking sites on the proteasome.

Phase transitions are important to understand cell dynamics, and the maturation of liquid droplets is relevant to neurodegenerative disorders. We combined NMR and Raman spectroscopies with microscopy to follow, over a period of days to months, droplet maturation of the protein fused in sarcoma (FUS). Our study reveals that the surface of the droplets plays a critical role in this process, while RNA binding prevents it. The maturation kinetics are faster in an agarose-stabilized biphasic sample compared with a monophasic condensed sample, owing to the larger surface-to-volume ratio. In addition, Raman spectroscopy reports structural differences upon maturation between the inside and the surface of droplets, which is comprised of β-sheet content, as revealed by solid-state NMR. In agreement with these observations, a solid crust-like shell is observed at the surface using microaspiration. Ultimately, matured droplets were converted into fibrils involving the prion-like domain as well as the first RGG motif.

Increasingly many studies reveal how ribosome composition can be tuned to optimally translate the transcriptome of individual cell types. In this study, we investigated the expression pattern, structure within the ribosome and effect on protein synthesis of the ribosomal protein paralog 39L (RPL39L). With a novel mass spectrometric approach we revealed the expression of RPL39L protein beyond mouse germ cells, in human pluripotent cells, cancer cell lines and tissue samples. We generated RPL39L knock-out mouse embryonic stem cell (mESC) lines and demonstrated that RPL39L impacts the dynamics of translation, to support the pluripotency and differentiation, spontaneous and along the germ cell lineage. Most differences in protein abundance between WT and RPL39L KO lines were explained by widespread autophagy. By CryoEM analysis of purified RPL39 and RPL39L-containing ribosomes we found that, unlike RPL39, RPL39L has two distinct conformations in the exposed segment of the nascent peptide exit tunnel, creating a distinct hydrophobic patch that has been predicted to support the efficient co-translational folding of alpha helices. Our study shows that ribosomal protein paralogs provide switchable modular components that can tune translation to the protein production needs of individual cell types.

The pyruvate dehydrogenase complex (PDHc) is a key megaenzyme linking glycolysis with the citric acid cycle. In mammalian PDHc, dihydrolipoamide acetyltransferase (E2) and the dihydrolipoamide dehydrogenase-binding protein (E3BP) form a 60-subunit core that associates with the peripheral subunits pyruvate dehydrogenase (E1) and dihydrolipoamide dehydrogenase (E3). The structure and stoichiometry of the fully assembled, mammalian PDHc or its core remained elusive. Here, we demonstrate that the human PDHc core is formed by 48 E2 copies that bind 48 E1 heterotetramers and 12 E3BP copies that bind 12 E3 homodimers. Cryo–electron microscopy, together with native and cross-linking mass spectrometry, confirmed a core model in which 8 E2 homotrimers and 12 E2-E2-E3BP heterotrimers assemble into a pseudoicosahedral particle such that the 12 E3BP molecules form six E3BP-E3BP intertrimer interfaces distributed tetrahedrally within the 60-subunit core. The even distribution of E3 subunits in the peripheral shell of PDHc guarantees maximum enzymatic activity of the megaenzyme.

Mechanisms by which G-patch activators tune the processive multi-tasking ATP-dependent RNA helicase Prp43 (DHX15 in humans) to productively remodel diverse RNA:protein complexes remain elusive. Here, a comparative study between a herein and previously characterized activators, Tma23 and Pxr1, respectively, defines segments that organize Prp43 function during ribosome assembly. In addition to the activating G-patch, we discover an inhibitory segment within Tma23 and Pxr1, I-patch, that restrains Prp43 ATPase activity. Cryo-electron microscopy and hydrogen-deuterium exchange mass spectrometry show how I-patch binds to the catalytic RecA-like domains to allosterically inhibit Prp43 ATPase activity. Tma23 and Pxr1 contain dimerization segments that organize Prp43 into higher-order complexes. We posit that Prp43 function at discrete locations on pre-ribosomal RNA is coordinated through toggling interactions with G-patch and I-patch segments. This could guarantee measured and timely Prp43 activation, enabling precise control over multiple RNA remodelling events occurring concurrently during ribosome formation. Portugal-Calisto et al report the discovery of a 14-residue inhibitory segment, I-patch, present within G-patch activators, Tma23 and Pxr1, that restrains the ATPase activity of the DEAH-RNA helicase Prp43 during ribosome assembly.

Stimulation of the innate immune system by foreign RNA elicits a potent response against invading pathogens and can trigger cell death. The mechanisms by which cells balance a robust response with cell-intrinsic lethality are still being uncovered. Employing genome-wide CRISPR-Cas9 genetic screens with triphosphorylated RNA stimulation, we identify speckled protein 110 (SP110) as a potent negative regulator of type 1 interferon-driven cell death. Death suppression by SP110 counteracts a death-promoting activity of another speckled protein, SP100. Both SP110 suppression and SP100 toxicity are mediated by direct interactions between the caspase activation and recruitment domains (CARDs) in each protein. SP100-induced death is mediated by homomeric CARD filaments that are disassembled by a heteromeric CARD interaction with SP110. Overexpression of SP100 is sufficient to overwhelm normal levels of SP110, leading to genotoxicity. Using cryo-EM and AlphaFold modeling, we develop and validate an atomic description of SP100 CARD filament formation and filament breaking by SP110. Genome-wide binding studies reveal that SP110 and SP100 normally associate at active promoters, but disruption of the CARD interaction releases SP100 to form toxic filaments. Overall, we uncover a novel regulatory partnership in human innate immunity that balances signal potency with cell intrinsic lethality. ### Competing Interest Statement The authors have declared no competing interest.

Amyloids are known as irreversible aggregates associated with neurodegenerative diseases. However, recent evidence shows that a subset of amyloids can form reversibly and fulfill essential cellular functions. Yet, the molecular mechanisms regulating functional amyloids and distinguishing them from pathological aggregates remain unclear. Here, we investigate the conserved principles of amyloid reversibility by studying the essential metabolic enzyme pyruvate kinase (PK) in yeast and human cells. We demonstrate that yeast PK (Cdc19) and human PK (PKM2) form reversible amyloids through a pH-sensitive amyloid core. Stress-induced cytosolic acidification promotes aggregation via protonation of specific glutamate (yeast) or histidine (human) residues within the amyloid core. Mutations mimicking protonation cause constitutive PK aggregation, while non-protonatable PK mutants remain soluble even upon stress. Physiological PK aggregation is coupled to metabolic rewiring and glycolysis arrest, causing severe growth defects when misregulated. Our work thus identifies an evolutionarily conserved, potentially widespread mechanism regulating functional amyloids during stress.

Physicians’ letters are the optimal source of diagnoses for registries. However, most registries demand for diagnosis codes such as ICD-10. We herein describe an algorithm that infers ICD-10 codes from German ophthalmologic physicians’ letters. We assess the method in three German eye hospitals. Our algorithm is based on the nearest-neighbor method as well as on a large thesaurus for ICD-10 codes. This thesaurus was embedded into a Word2Vec space created from anonymized physicians’ reports of the first hospital. For evaluation, each of the three hospitals sent all diagnoses taken from 100 letters. The inferred ICD-10 codes were evaluated for correctness by the senders. A total of 3332 natural language terms had been sent in (812 hospital one, 1473 hospital two, 1047 hospital three). A total of 526 non-diagnoses were excluded upfront. 2806 ICD-10 codes were inferred (771 hospital one, 1226 hospital two, 809 hospital three). In the first hospital, 98% were fully correct and 99% correct at the level of the superordinate disease concept. The percentages in hospital two were 69% and 86%. The respective numbers for hospital three were 69% and 91%. Our simple method is capable of inferring ICD-10 codes for German natural language diagnoses, especially when the embedding space has been built with physicians’ letters from the same hospital. The method may yield sufficient accuracy for many tasks in the multi-centric setting and can easily be adapted to other languages/specialities.

The discovery of nackednaviruses provided new insight into the evolutionary history of the hepatitis B virus (HBV): The common ancestor of HBV and nackednaviruses was non-enveloped and while HBV acquired an envelope during evolution, nackednaviruses remained non-enveloped. We report the capsid structure of the African cichlid nackednavirus (ACNDV), determined by cryo-EM at 3.7 Å resolution. This enables direct comparison with the known capsid structures of HBV and duck HBV, prototypic representatives of the mammalian and avian lineages of the enveloped Hepadnaviridae, respectively. The sequence identity with HBV is 24% and both the ACNDV capsid protein fold and the capsid architecture are very similar to those of the Hepadnaviridae and HBV in particular. Acquisition of the hepadnaviral envelope was thus not accompanied by a major change in capsid structure. Dynamic residues at the spike tip are tentatively assigned by solid-state NMR, while the C-terminal domain is invisible due to dynamics. Solid-state NMR characterization of the capsid structure reveals few conformational differences between the quasi-equivalent subunits of the ACNDV capsid and an overall higher capsid structural disorder compared to HBV. Despite these differences, the capsids of ACNDV and HBV are structurally highly similar despite the 400 million years since their separation.

Amyloids were long viewed as irreversible, pathological aggregates, often associated with neurodegenerative diseases1. However, recent insights challenge this view, providing evidence that reversible amyloids can form upon stress conditions and fulfil crucial cellular functions2. Yet, the molecular mechanisms regulating functional amyloids and the differences to their pathological counterparts remain poorly understood. Here we investigate the conserved principles of amyloid reversibility by studying the essential metabolic enzyme pyruvate kinase (PK) in yeast and human cells. We demonstrate that PK forms stress-dependent reversible amyloids through a pH-sensitive amyloid core. Stress- induced cytosolic acidification promotes aggregate formation via protonation of specific glutamate (in yeast) or histidine (in human) residues within the amyloid core. Our work thus unravels a conserved and potentially widespread mechanism underlying amyloid functionality and reversibility, fine-tuned to the respective physiological cellular pH range.

Mitochondrial ribosomes (mitoribosomes) play a central role in synthesizing mitochondrial inner membrane proteins responsible for oxidative phosphorylation. Although mitoribosomes from different organisms exhibit considerable structural variations, recent insights into mitoribosome assembly suggest that mitoribosome maturation follows common principles and involves a number of conserved assembly factors. To investigate the steps involved in the assembly of the mitoribosomal small subunit (mt-SSU) we determined the cryoelectron microscopy structures of middle and late assembly intermediates of the Trypanosoma brucei mitochondrial small subunit (mt-SSU) at 3.6- and 3.7-Å resolution, respectively. We identified five additional assembly factors that together with the mitochondrial initiation factor 2 (mt-IF-2) specifically interact with functionally important regions of the rRNA, including the decoding center, thereby preventing premature mRNA or large subunit binding. Structural comparison of assembly intermediates with mature mt-SSU combined with RNAi experiments suggests a noncanonical role of mt-IF-2 and a stepwise assembly process, where modular exchange of ribosomal proteins and assembly factors together with mt-IF-2 ensure proper 9S rRNA folding and protein maturation during the final steps of assembly.

Heterogenous nuclear ribonucleoproteins (hnRNPs) are abundant proteins implicated in various steps of RNA processing that assemble on nuclear RNA into larger complexes termed 40S hnRNP particles. Despite their initial discovery 55 years ago, our understanding of these intriguing macromolecular assemblies remains limited. Here, we report the biochemical purification of native 40S hnRNP particles and the determination of their complete protein composition by label-free quantitative mass spectrometry, identifying A-group and C-group hnRNPs as the major protein constituents. Isolated 40S hnRNP particles dissociate upon RNA digestion and can be reconstituted in vitro on defined RNAs in the presence of the individual protein components, demonstrating a scaffolding role for RNA in nucleating particle formation. Finally, we revealed their nanometer scale, condensate-like nature, promoted by intrinsically disordered regions of A-group hnRNPs. Collectively, we identify nuclear 40S hnRNP particles as novel dynamic biomolecular condensates.

Abstract Biogenesis of ribosomal subunits involves enzymatic modifications of rRNA that fine-tune functionally important regions. The universally conserved prokaryotic dimethyltransferase KsgA sequentially modifies two universally conserved adenosine residues in helix 45 of the small ribosomal subunit rRNA, which is in proximity of the decoding site. Here we present the cryo-EM structure of Escherichia coli KsgA bound to an E. coli 30S at a resolution of 3.1 Å. The high-resolution structure reveals how KsgA recognizes immature rRNA and binds helix 45 in a conformation where one of the substrate nucleotides is flipped-out into the active site. We suggest that successive processing of two adjacent nucleotides involves base-flipping of the rRNA, which allows modification of the second substrate nucleotide without dissociation of the enzyme. Since KsgA is homologous to the essential eukaryotic methyltransferase Dim1 involved in 40S maturation, these results have also implications for understanding eukaryotic ribosome maturation.

AbstractZelluläres Leben erfordert ein hohes Maß an molekularer Komplexität und Selbstorganisation, von denen einige in einem präbiotischen Kontext entstanden sein müssen. Hier demonstrieren wir, wie beide dieser Eigenschaften in einem plausibel präbiotischen System entstehen können. Wir fanden heraus, dass chemische Gradienten in einfachen Mischungen von aktivierten Aminosäuren und Fettsäuren zur Bildung von amyloidartigen Peptidfibrillen führen können, die innerhalb eines protozellulären Kompartiments lokalisiert sind. Bei diesem Prozess wirken die Fettsäure‐ oder Lipidvesikel sowohl als Filter, der die selektive Passage von aktivierten Aminosäuren ermöglicht, als auch als Barriere, welche die Diffusion der amyloidogenen Peptide blockiert, die sich spontan innerhalb der Vesikel bilden. Diese Synergie zwischen zwei unterschiedlichen Bausteinen des Lebens induziert eine signifikante Zunahme der molekularen Komplexität und räumlichen Ordnung und bietet damit einen Weg für die frühe molekulare Evolution, die zu einer lebenden Zelle führen könnte.

Cellular life requires a high degree of molecular complexity and self-organization, some of which must have originated in a prebiotic context. Here, we demonstrate how both of these features can emerge in a plausibly prebiotic system. We found that chemical gradients in simple mixtures of activated amino acids and fatty acids can lead to the formation of amyloid-like peptide fibrils that are localized inside of a proto-cellular compartment. In this process, the fatty acid or lipid vesicles act both as a filter, allowing the selective passage of activated amino acids, and as a barrier, blocking the diffusion of the amyloidogenic peptides that form spontaneously inside the vesicles. This synergy between two distinct building blocks of life induces a significant increase in molecular complexity and spatial order thereby providing a route for the early molecular evolution that could give rise to a living cell.

Abstract U1 small nuclear ribonucleoparticle (U1 snRNP) plays a central role during RNA processing. Previous structures of U1 snRNP revealed how the ribonucleoparticle is organized and recognizes the pre-mRNA substrate at the exon–intron junction. As with many other ribonucleoparticles involved in RNA metabolism, U1 snRNP contains extensions made of low complexity sequences. Here, we developed a protocol to reconstitute U1 snRNP in vitro using mostly full-length components in order to perform liquid-state NMR spectroscopy. The accuracy of the reconstitution was validated by probing the shape and structure of the particle by SANS and cryo-EM. Using an NMR spectroscopy-based approach, we probed, for the first time, the U1 snRNP tails at atomic detail and our results confirm their high degree of flexibility. We also monitored the labile interaction between the splicing factor PTBP1 and U1 snRNP and validated the U1 snRNA stem loop 4 as a binding site for the splicing regulator on the ribonucleoparticle. Altogether, we developed a method to probe the intrinsically disordered regions of U1 snRNP and map the interactions controlling splicing regulation. This approach could be used to get insights into the molecular mechanisms of alternative splicing and screen for potential RNA therapeutics.