Bacterial histone HBb from Bdellovibrio bacteriovorus compacts DNA by bending.

Histones are essential for genome compaction and transcription regulation in eukaryotes, where they assemble into octamers to form the nucleosome core. In contrast, archaeal histones assemble into dimers that form hypernucleosomes upon DNA binding. Although histone homologs have been identified in bacteria recently, their DNA-binding characteristics remain largely unexplored. Our study reveals that the bacterial histone HBb (Bd0055) is indispensable for the survival of Bdellovibrio bacteriovorus, suggesting critical roles in DNA organization and gene regulation. By determining crystal structures of free and DNA-bound HBb, we unveil its distinctive dimeric assembly, diverging from those of eukaryotic and archaeal histones, while also elucidating how it binds and bends DNA through interaction interfaces reminiscent of eukaryotic and archaeal histones. Building on this, by employing various biophysical and biochemical approaches, we further substantiated the ability of HBb to bind and compact DNA by bending in a sequence-independent manner. Finally, using DNA affinity purification and sequencing, we reveal that HBb binds along the entire genomic DNA of B. bacteriovorus without sequence specificity. These distinct DNA-binding properties of bacterial histones, showcasing remarkable similarities yet significant differences from their archaeal and eukaryotic counterparts, highlight the diverse roles histones play in DNA organization across all domains of life.

Histones, traditionally known for organizing and regulating DNA in eukaryotes and archaea, have recently been discovered in bacteria, opening up a new frontier in our understanding of genome organization across the domains of life. Our study investigates the largely unexplored DNA-binding properties of bacterial histones, focusing on HBb in Bdellovibrio bacteriovorus. We reveal that HBb is essential for bacterial survival and exhibits DNA-binding properties similar to archaeal and eukaryotic histones. However, unlike eukaryotic and archaeal histones, which wrap DNA, HBb bends DNA without sequence specificity. This work not only broadens our understanding of DNA organization across different life forms but also suggests that bacterial histones may have diverse roles in genome organization.

A computational model for lipid-anchored polysaccharide export by the outer membrane protein GfcD.

Many bacteria are protected by different types of polysaccharide capsules, structures formed of long repetitive glycan chains that are sometimes free and sometimes anchored to the outer membrane via lipid tails. One type, called group 4 capsule, results from the expression of the gfcABCDE-etp-etk operon in Escherichia coli. Of the proteins encoded in this operon, GfcE is thought to provide the export pore for free polysaccharide chains, but none of the proteins has been implicated in the export of chains carrying a lipid anchor. For this function, GfcD has been a focus of attention as the only outer membrane ß-barrel encoded in the operon. AlphaFold predicts two ß-barrel domains in GfcD, a canonical N-terminal one of 12 strands and an unusual C-terminal one of 13 strands, which features a large lateral aperture between strands ß1 and ß13. This immediately suggests a lateral exit gate for hydrophobic molecules into the membrane, analogous to the one proposed for the lipopolysaccharide export pore LptD. Here, we report an unsteered molecular dynamics study of GfcD embedded in the bacterial outer membrane, with the common polysaccharide anchor, lipid A, inserted in the pore of the C-terminal barrel. Our results show that the lateral aperture does not collapse during simulations and membrane lipids nevertheless do not penetrate the barrel but the lipid chains of the lipid A molecule readily exit into the membrane.

Gram-negative outer-membrane proteins with multiple ß-barrel domains.

Outer-membrane beta barrels (OMBBs) are found in the outer membrane of gram-negative bacteria and eukaryotic organelles. OMBBs fold as antiparallel ß-sheets that close onto themselves, forming pores that traverse the membrane. Currently known structures include only one barrel, of 8 to 36 strands, per chain. The lack of multi-OMBB chains is surprising, as most OMBBs form oligomers, and some function only in this state. Using a combination of sensitive sequence comparison methods and coevolutionary analysis tools, we identify many proteins combining multiple beta barrels within a single chain; combinations that include eight-stranded barrels prevail. These multibarrels seem to be the result of independent, lineage-specific fusion and amplification events. The absence of multibarrels that are universally conserved in bacteria with an outer membrane, coupled with their frequent de novo genesis, suggests that their functions are not essential but rather beneficial in specific environments. Adjacent barrels of complementary function within the same chain may allow for functions beyond those of the individual barrels.

Archaeal Connectase is a specific and efficient protein ligase related to proteasome ß subunits.

Sequence-specific protein ligations are widely used to produce customized proteins "on demand." Such chimeric, immobilized, fluorophore-conjugated or segmentally labeled proteins are generated using a range of chemical, (split) intein, split domain, or enzymatic methods. Where short ligation motifs and good chemoselectivity are required, ligase enzymes are often chosen, although they have a number of disadvantages, for example poor catalytic efficiency, low substrate specificity, and side reactions. Here, we describe a sequence-specific protein ligase with more favorable characteristics. This ligase, Connectase, is a monomeric homolog of 20S proteasome subunits in methanogenic archaea. In pulldown experiments with Methanosarcina mazei cell extract, we identify a physiological substrate in methyltransferase A (MtrA), a key enzyme of archaeal methanogenesis. Using microscale thermophoresis and X-ray crystallography, we show that only a short sequence of about 20 residues derived from MtrA and containing a highly conserved KDPGA motif is required for this high-affinity interaction. Finally, in quantitative activity assays, we demonstrate that this recognition tag can be repurposed to allow the ligation of two unrelated proteins. Connectase catalyzes such ligations at substantially higher rates, with higher yields, but without detectable side reactions when compared with a reference enzyme. It thus presents an attractive tool for the development of new methods, for example in the preparation of selectively labeled proteins for NMR, the covalent and geometrically defined attachment of proteins on surfaces for cryo-electron microscopy, or the generation of multispecific antibodies.

New protein families with hendecad coiled coils in the proteome of life.

Coiled coils are a widespread and well understood protein fold. Their short and simple repeats underpin considerable structural and functional diversity. The vast majority of coiled coils consist of 7-residue (heptad) sequence repeats, but in essence most combinations of 3- and 4-residue segments, each starting with a residue of the hydrophobic core, are compatible with coiled-coil structure. The most frequent among these other repeat patterns are 11-residue (hendecad, 3 + 4 + 4) repeats. Hendecads are frequently found in low copy number, interspersed between heptads, but some proteins consist largely or entirely of hendecad repeats. Here we describe the first large-scale survey of these proteins in the proteome of life. For this, we scanned the protein sequence database for sequences with 11-residue periodicity that lacked ß-strand prediction. We then clustered these by pairwise similarity to construct a map of potential hendecad coiled-coil families. Here we discuss these according to their structural properties, their potential cellular roles, and the evolutionary mechanisms shaping their diversity. We note in particular the continuous amplification of hendecads, both within existing proteins and de novo from previously non-coding sequence, as a powerful mechanism in the genesis of new coiled-coil forms.

Design of novel granulopoietic proteins by topological rescaffolding.

Computational protein design is rapidly becoming more powerful, and improving the accuracy of computational methods would greatly streamline protein engineering by eliminating the need for empirical optimization in the laboratory. In this work, we set out to design novel granulopoietic agents using a rescaffolding strategy with the goal of achieving simpler and more stable proteins. All of the 4 experimentally tested designs were folded, monomeric, and stable, while the 2 determined structures agreed with the design models within less than 2.5 Å. Despite the lack of significant topological or sequence similarity to their natural granulopoietic counterpart, 2 designs bound to the granulocyte colony-stimulating factor (G-CSF) receptor and exhibited potent, but delayed, in vitro proliferative activity in a G-CSF-dependent cell line. Interestingly, the designs also induced proliferation and differentiation of primary human hematopoietic stem cells into mature granulocytes, highlighting the utility of our approach to develop highly active therapeutic leads purely based on computational design.

The VCBS superfamily forms a third supercluster of ß-propellers that includes tachylectin and integrins.

MOTIVATION: ß-Propellers are found in great variety across all kingdoms of life. They assume many cellular roles, primarily as scaffolds for macromolecular interactions and catalysis. Despite their diversity, most ß-propeller families clearly originated by amplification from the same ancient peptide-the 'blade'. In cluster analyses, ß-propellers of the WD40 superfamily always formed the largest group, to which some important families, such as the α-integrin, Asp-box and glycoside hydrolase ß-propellers connected weakly. Motivated by the dramatic growth of sequence databases we revisited these connections, with a special focus on VCBS-like ß-propellers, which have not been analysed for their evolutionary relationships so far. RESULTS: We found that VCBS-like form a supercluster with integrin-like ß-propellers and tachylectins, clearly delimited from the superclusters formed by WD40 and Asp-Box ß-propellers. Connections between the three superclusters are made mainly through PQQ-like ß-propeller. Our results present a new, greatly expanded view of the ß-propeller classification landscape. SUPPLEMENTARY INFORMATION: Supplementary data are available at Bioinformatics online.

An astonishing wealth of new proteasome homologs.

MOTIVATION: The proteasome is the main proteolytic machine for targeted protein degradation in archaea and eukaryotes. While some bacteria also possess the proteasome, most of them contain a simpler and more specialized homolog, the heat shock locus V protease. In recent years, three further homologs of the proteasome core subunits have been characterized in prokaryotes: Anbu, BPH and connectase. With the inclusion of these members, the family of proteasome-like proteins now exhibits a range of architectural and functional forms, from the canonical proteasome, a barrel-shaped protease without pronounced intrinsic substrate specificity, to the monomeric connectase, a highly specific protein ligase. RESULTS: We employed systematic sequence searches to show that we have only seen the tip of the iceberg so far and that beyond the hitherto known proteasome homologs lies a wealth of distantly related, uncharacterized homologs. We describe a total of 22 novel proteasome homologs in bacteria and archaea. Using sequence and structure analysis, we analyze their evolutionary history and assess structural differences that may modulate their function. With this initial description, we aim to stimulate the experimental investigation of these novel proteasome-like family members. AVAILABILITY AND IMPLEMENTATION: The protein sequences in this study are searchable in the MPI Bioinformatics Toolkit (https://toolkit.tuebingen.mpg.de) with ProtBLAST/PSI-BLAST and with HHpred (database 'proteasome_homologs'). The following data are available at https://data.mendeley.com/datasets/t48yhff7hs/3: (i) sequence alignments for each proteasome-like homolog, (ii) the coordinates for their structural models and (iii) a cluster-map file, which can be navigated interactively in CLANS and gives direct access to all the sequences in this study. SUPPLEMENTARY INFORMATION: Supplementary data are available at Bioinformatics online.

An endosomal tether undergoes an entropic collapse to bring vesicles together.

An early step in intracellular transport is the selective recognition of a vesicle by its appropriate target membrane, a process regulated by Rab GTPases via the recruitment of tethering effectors. Membrane tethering confers higher selectivity and efficiency to membrane fusion than the pairing of SNAREs (soluble N-ethylmaleimide-sensitive factor attachment protein receptors) alone. Here we address the mechanism whereby a tethered vesicle comes closer towards its target membrane for fusion by reconstituting an endosomal asymmetric tethering machinery consisting of the dimeric coiled-coil protein EEA1 (refs 6, 7) recruited to phosphatidylinositol 3-phosphate membranes and binding vesicles harbouring Rab5. Surprisingly, structural analysis reveals that Rab5:GTP induces an allosteric conformational change in EEA1, from extended to flexible and collapsed. Through dynamic analysis by optical tweezers, we confirm that EEA1 captures a vesicle at a distance corresponding to its extended conformation, and directly measure its flexibility and the forces induced during the tethering reaction. Expression of engineered EEA1 variants defective in the conformational change induce prominent clusters of tethered vesicles in vivo. Our results suggest a new mechanism in which Rab5 induces a change in flexibility of EEA1, generating an entropic collapse force that pulls the captured vesicle towards the target membrane to initiate docking and fusion.

The breakthrough in protein structure prediction.

Proteins are the essential agents of all living systems. Even though they are synthesized as linear chains of amino acids, they must assume specific three-dimensional structures in order to manifest their biological activity. These structures are fully specified in their amino acid sequences - and therefore in the nucleotide sequences of their genes. However, the relationship between sequence and structure, known as the protein folding problem, has remained elusive for half a century, despite sustained efforts. To measure progress on this problem, a series of doubly blind, biennial experiments called CASP (critical assessment of structure prediction) were established in 1994. We were part of the assessment team for the most recent CASP experiment, CASP14, where we witnessed an astonishing breakthrough by DeepMind, the leading artificial intelligence laboratory of Alphabet Inc. The models filed by DeepMind's structure prediction team using the program AlphaFold2 were often essentially indistinguishable from experimental structures, leading to a consensus in the community that the structure prediction problem for single protein chains has been solved. Here, we will review the path to CASP14, outline the method employed by AlphaFold2 to the extent revealed, and discuss the implications of this breakthrough for the life sciences.

Coiled Coils - A Model System for the 21st Century.

α-Helical coiled coils were described more than 60 years ago as simple, repetitive structures mediating oligomerization and mechanical stability. Over the past 20 years, however, they have emerged as one of the most diverse protein folds in nature, enabling many biological functions beyond mechanical rigidity, such as membrane fusion, signal transduction, and solute transport. Despite this great diversity, their structures can be described by parametric equations, making them uniquely suited for rational protein design. Far from having been exhausted as a source of structural insight and a basis for functional engineering, coiled coils are poised to become even more important for protein science in the coming decades.

High-accuracy protein structure prediction in CASP14.

The application of state-of-the-art deep-learning approaches to the protein modeling problem has expanded the "high-accuracy" category in CASP14 to encompass all targets. Building on the metrics used for high-accuracy assessment in previous CASPs, we evaluated the performance of all groups that submitted models for at least 10 targets across all difficulty classes, and judged the usefulness of those produced by AlphaFold2 (AF2) as molecular replacement search models with AMPLE. Driven by the qualitative diversity of the targets submitted to CASP, we also introduce DipDiff as a new measure for the improvement in backbone geometry provided by a model versus available templates. Although a large leap in high-accuracy is seen due to AF2, the second-best method in CASP14 out-performed the best in CASP13, illustrating the role of community-based benchmarking in the development and evolution of the protein structure prediction field.

Assessing the utility of CASP14 models for molecular replacement.

The assessment of CASP models for utility in molecular replacement is a measure of their use in a valuable real-world application. In CASP7, the metric for molecular replacement assessment involved full likelihood-based molecular replacement searches; however, this restricted the assessable targets to crystal structures with only one copy of the target in the asymmetric unit, and to those where the search found the correct pose. In CASP10, full molecular replacement searches were replaced by likelihood-based rigid-body refinement of models superimposed on the target using the LGA algorithm, with the metric being the refined log-likelihood-gain (LLG) score. This enabled multi-copy targets and very poor models to be evaluated, but a significant further issue remained: the requirement of diffraction data for assessment. We introduce here the relative-expected-LLG (reLLG), which is independent of diffraction data. This reLLG is also independent of any crystal form, and can be calculated regardless of the source of the target, be it X-ray, NMR or cryo-EM. We calibrate the reLLG against the LLG for targets in CASP14, showing that it is a robust measure of both model and group ranking. Like the LLG, the reLLG shows that accurate coordinate error estimates add substantial value to predicted models. We find that refinement by CASP groups can often convert an inadequate initial model into a successful MR search model. Consistent with findings from others, we show that the AlphaFold2 models are sufficiently good, and reliably so, to surpass other current model generation strategies for attempting molecular replacement phasing.

Computational models in the service of X-ray and cryo-electron microscopy structure determination.

Critical assessment of structure prediction (CASP) conducts community experiments to determine the state of the art in computing protein structure from amino acid sequence. The process relies on the experimental community providing information about not yet public or about to be solved structures, for use as targets. For some targets, the experimental structure is not solved in time for use in CASP. Calculated structure accuracy improved dramatically in this round, implying that models should now be much more useful for resolving many sorts of experimental difficulties. To test this, selected models for seven unsolved targets were provided to the experimental groups. These models were from the AlphaFold2 group, who overall submitted the most accurate predictions in CASP14. Four targets were solved with the aid of the models, and, additionally, the structure of an already solved target was improved. An a posteriori analysis showed that, in some cases, models from other groups would also be effective. This paper provides accounts of the successful application of models to structure determination, including molecular replacement for X-ray crystallography, backbone tracing and sequence positioning in a cryo-electron microscopy structure, and correction of local features. The results suggest that, in future, there will be greatly increased synergy between computational and experimental approaches to structure determination.

Target highlights in CASP14: Analysis of models by structure providers.

The biological and functional significance of selected Critical Assessment of Techniques for Protein Structure Prediction 14 (CASP14) targets are described by the authors of the structures. The authors highlight the most relevant features of the target proteins and discuss how well these features were reproduced in the respective submitted predictions. The overall ability to predict three-dimensional structures of proteins has improved remarkably in CASP14, and many difficult targets were modeled with impressive accuracy. For the first time in the history of CASP, the experimentalists not only highlighted that computational models can accurately reproduce the most critical structural features observed in their targets, but also envisaged that models could serve as a guidance for further studies of biologically-relevant properties of proteins.

The evolution of the huntingtin-associated protein 40 (HAP40) in conjunction with huntingtin.

BACKGROUND: The huntingtin-associated protein 40 (HAP40) abundantly interacts with huntingtin (HTT), the protein that is altered in Huntington's disease (HD). Therefore, we analysed the evolution of HAP40 and its interaction with HTT. RESULTS: We found that in amniotes HAP40 is encoded by a single-exon gene, whereas in all other organisms it is expressed from multi-exon genes. HAP40 co-occurs with HTT in unikonts, including filastereans such as Capsaspora owczarzaki and the amoebozoan Dictyostelium discoideum, but both proteins are absent from fungi. Outside unikonts, a few species, such as the free-living amoeboflagellate Naegleria gruberi, contain putative HTT and HAP40 orthologs. Biochemically we show that the interaction between HTT and HAP40 extends to fish, and bioinformatic analyses provide evidence for evolutionary conservation of this interaction. The closest homologue of HAP40 in current protein databases is the family of soluble N-ethylmaleimide-sensitive factor attachment proteins (SNAPs). CONCLUSION: Our results indicate that the transition from a multi-exon to a single-exon gene appears to have taken place by retroposition during the divergence of amphibians and amniotes, followed by the loss of the parental multi-exon gene. Furthermore, it appears that the two proteins probably originated at the root of eukaryotes. Conservation of the interaction between HAP40 and HTT and their likely coevolution strongly indicate functional importance of this interaction.

Histones predate the split between bacteria and archaea.

MOTIVATION: Histones form octameric complexes called nucleosomes, which organize the genomic DNA of eukaryotes into chromatin. Each nucleosome comprises two copies each of the histones H2A, H2B, H3 and H4, which share a common ancestry. Although histones were initially thought to be a eukaryotic innovation, the subsequent identification of archaeal homologs led to the notion that histones emerged before the divergence of archaea and eukaryotes. RESULTS: Here, we report the detection and classification of two new groups of histone homologs, which are present in both archaea and bacteria. Proteins in one group consist of two histone subunits welded into single-chain pseudodimers, whereas in the other they resemble eukaryotic core histone subunits and show sequence patterns characteristic of DNA binding. The sequences come from a broad spectrum of deeply-branching lineages, excluding their genesis by horizontal gene transfer. Our results extend the origin of histones to the last universal common ancestor. SUPPLEMENTARY INFORMATION: Supplementary data are available at Bioinformatics online.

A secreted fungal histidine- and alanine-rich protein regulates metal ion homeostasis and oxidative stress.

Like pathogens, beneficial endophytic fungi secrete effector proteins to promote plant colonization, for example, through perturbation of host immunity. The genome of the root endophyte Serendipita indica encodes a novel family of highly similar, small alanine- and histidine-rich proteins, whose functions remain unknown. Members of this protein family carry an N-terminal signal peptide and a conserved C-terminal DELD motif. Here we report on the functional characterization of the plant-responsive DELD family protein Dld1 using a combination of structural, biochemical, biophysical and cytological analyses. The crystal structure of Dld1 shows an unusual, monomeric histidine zipper consisting of two antiparallel coiled-coil helices. Similar to other histidine-rich proteins, Dld1 displays varying affinity to different transition metal ions and undergoes metal ion- and pH-dependent unfolding. Transient expression of mCherry-tagged Dld1 in barley leaf and root tissue suggests that Dld1 localizes to the plant cell wall and accumulates at cell wall appositions during fungal penetration. Moreover, recombinant Dld1 enhances barley root colonization by S. indica, and inhibits H2 O2 -mediated radical polymerization of 3,3'-diaminobenzidine. Our data suggest that Dld1 has the potential to enhance micronutrient accessibility for the fungus and to interfere with oxidative stress and reactive oxygen species homeostasis to facilitate host colonization.

The ancestral KH peptide at the root of a domain family with three different folds.

Motivation: The direct ancestor of the DNA-protein world of today is considered to have been an RNA-peptide world, in which peptides were co-factors of RNA-mediated catalysis and replication. Evidence for these ancestral peptides, from which folded proteins evolved, can be derived even today from regions of local sequence similarity within globally dissimilar folds. One of these is the 45-residue motif common to both folds of the hnRNP K homology (KH) domain. Results: In a survey of KH domains, we found a third fold that contains the KH motif at its core. This corresponds to the Small Domain of bacterial Ribonucleases G/E and, like type I and type II KH domains, it cannot be related to the others by a single genetic event, providing further support for the KH motif as an ancestral peptide predating folded proteins. Supplementary information: Supplementary data are available at Bioinformatics online.

Structural diversity of coiled coils in protein fibers of the bacterial cell envelope.

The cell envelope of bacteria shows great diversity in architecture and composition, to a large extent due to its proteome. Proteins localized to the cell envelope, whether integrally embedded in the membrane, membrane-anchored, or peripherally associated as part of a macromolecular complex, often form elongated fibers, in which coiled coils represent a prominent structural element. These coiled-coil segments show a surprising degree of structural variability, despite being shaped by a small number of simple biophysical rules, foremost being their geometry of interaction referred to as 'knobs-into-holes'. Here we will review this diversity, particularly as it has emerged over the last decade.