Stabilization of F-actin by Salmonella effector SipA resembles the structural effects of inorganic phosphate and phalloidin

Entry of Salmonella into host enterocytes relies on its pathogenicity island 1 effector SipA. We found that SipA binds to F-actin in a 1:2 stoichiometry with sub-nanomolar affinity. A cryo-EM reconstruction revealed that SipA's globular core binds at the groove between actin strands, whereas the extended C-terminal arm penetrates deeply into the inter-strand space, stabilizing F-actin from within. The unusually strong binding of SipA is achieved by a combination of fast association via the core and very slow dissociation dictated by the arm. Similar to Pi, BeF3, and phalloidin, SipA potently inhibited actin depolymerization by actin depolymerizing factor (ADF)/cofilin, which correlated with increased filament stiffness, supporting the hypothesis that F-actin's mechanical properties contribute to the recognition of its nucleotide state by protein partners. The remarkably strong binding to F-actin maximizes the toxin's effects at the injection site while minimizing global influence on the cytoskeleton and preventing pathogen detection by the host cell.

Structural basis for the phase separation of the chromosome passenger complex

The physical basis of phase separation is thought to consist of the same types of bonds that specify conventional macromolecular interactions yet is unsatisfyingly often referred to as 'fuzzy'. Gaining clarity on the biogenesis of membraneless cellular compartments is one of the most demanding challenges in biology. Here, we focus on the chromosome passenger complex (CPC), that forms a chromatin body that regulates chromosome segregation in mitosis. Within the three regulatory subunits of the CPC implicated in phase separation - a heterotrimer of INCENP, Survivin, and Borealin - we identify the contact regions formed upon droplet formation using hydrogen/deuterium exchange mass spectrometry (HXMS). These contact regions correspond to some of the interfaces seen between individual heterotrimers within the crystal lattice they form. A major contribution comes from specific electrostatic interactions that can be broken and reversed through initial and compensatory mutagenesis, respectively. Our findings reveal structural insight for interactions driving liquid-liquid demixing of the CPC. Moreover, we establish HXMS as an approach to define the structural basis for phase separation.

Chromosomal passenger complex condensates generate parallel microtubule bundles in vitro

The mitotic spindle contains many bundles of microtubules (MTs) including midzones and kinetochore fibers, but little is known about how bundled structures are formed. Here, we show that the chromosomal passenger complex (CPC) purified from Escherichia coli undergoes liquid-liquid demixing in vitro. An emergent property of the resultant condensates is to generate parallel MT bundles when incubated with free tubulin and GTP in vitro. We demonstrate that MT bundles emerge from CPC droplets with protruding minus ends that then grow into long and tapered MT structures. During this growth, we found that the CPC in these condensates apparently reorganize to coat and bundle the resulting MT structures. CPC mutants attenuated for liquid-liquid demixing or MT binding prevented the generation of parallel MT bundles in vitro and reduced the number of MTs present at spindle midzones in HeLa cells. Our data demonstrate that an in vitro biochemical activity to produce MT bundles emerges after the concentration of the CPC and provides models for how cells generate parallel-bundled MT structures that are important for the assembly of the mitotic spindle. Moreover, these data suggest that cells contain MT-organizing centers that generate MT bundles that emerge with the opposite polarity from centrosomes.

Stabilization of F-actin by Salmonella effector SipA resembles the structural effects of inorganic phosphate and phalloidin

Entry of Salmonella into host enterocytes strictly relies on its pathogenicity island 1 effector SipA. We found that SipA binds to F-actin in a unique mode in a 1:2 stoichiometry with picomolar affinity. A cryo-EM reconstruction revealed that SipA's globular core binds at the grove between actin strands, whereas the extended C-terminal arm penetrates deeply into the inter-strand space, stabilizing F-actin from within. The unusually strong binding of SipA is achieved via a combination of fast association via the core and very slow dissociation dictated by the arm. Similarly to Pi, BeF3, and phalloidin, SipA potently inhibited actin depolymerization by ADF/cofilin, which correlated with the increased filament stiffness, supporting the hypothesis that F-actin's mechanical properties contribute to the recognition of its nucleotide state by protein partners. The remarkably strong binding to F-actin maximizes the toxin's effects at the injection site while minimizing global influence on the cytoskeleton and preventing pathogen detection by the host cell.

Structural Basis for the Phase Separation of the Chromosome Passenger Complex

The physical basis of phase separation is thought to consist of the same types of bonds that specify conventional macromolecular interactions yet is unsatisfyingly often referred to as 'fuzzy'. Gaining clarity on the biogenesis of membraneless cellular compartments is one of the most demanding challenges in biology. Here, we focus on the chromosome passenger complex (CPC), that forms a chromatin body that regulates chromosome segregation in mitosis. Within the three regulatory subunits of the CPC implicated in phase separation - a heterotrimer of INCENP, Survivin, and Borealin - we identify the contact regions formed upon droplet formation using hydrogen/deuterium-exchange mass spectrometry (HXMS). These contact regions correspond to some of the interfaces seen between individual heterotrimers within the crystal lattice they form. A major contribution comes from specific electrostatic interactions that can be broken and reversed through initial and compensatory mutagenesis, respectively. Our findings reveal structural insight for interactions driving liquid-liquid demixing of the CPC. Moreover, we establish HXMS as an approach to define the structural basis for phase separation.

Tip60 acetylation of histone H3K4 temporally controls chromosome passenger complex localization

The Chromosome Passenger Complex (CPC) generates chromosome autonomous signals that regulate mitotic events critical for genome stability. Tip60 is a lysine acetyltransferase that is a tumor suppressor and is targeted for proteasomal degradation by oncogenic papilloma viruses. Mitotic regulation requires the localization of the CPC to inner centromeres, which is driven by the Haspin kinase phosphorylating histone H3 on threonine 3 (H3T3ph). Here we describe how Tip60 acetylates histone H3 at lysine 4 (H3K4ac) to block both the H3T3ph writer and the reader to ensure that this mitotic signaling cannot begin before prophase. Specifically, H3K4ac inhibits Haspin phosphorylation of H3T3 and prevents binding of the Survivin subunit to H3T3ph. Tip60 acetylates H3K4 during S/G2 at centromeres. Inhibition of Tip60 allows the CPC to bind centromeres in G2 cells, and targeting of Tip60 to centromeres prevents CPC localization in mitosis. The H3K4ac mark is removed in prophase by HDAC3 to initiate the CPC localization cascade. Together, our results suggest that Tip60 and HDAC3 temporally control H3K4 acetylation to precisely time the targeting of the CPC to inner centromeres.

Regioselectivity of hyoscyamine 6β-hydroxylase-catalysed hydroxylation as revealed by high-resolution structural information and QM/MM calculations

Hyoscyamine 6β-hydroxylase (H6H) is a bifunctional non-heme 2-oxoglutarate/Fe2+-dependent dioxygenase that catalyzes the two final steps in the biosynthesis of scopolamine. Based on high resolution crystal structures of H6H from Datura metel, detailed information on substrate binding was obtained that provided insights into the onset of the enzymatic process. In particular, the role of two prominent residues was revealed - Glu-116 that interacts with the tertiary amine located on the hyoscyamine tropane moiety and Tyr-326 that forms CH-π hydrogen bonds with the hyoscyamine phenyl ring. The structures were used as the basis for QM/MM calculations that provided an explanation for the regioselectivity of the hydroxylation reaction on the hyoscyamine tropane moiety (C6 vs. C7) and quantified contributions of active site residues to respective barrier heights.

A study on the structure, mechanism, and biochemistry of kanamycin B dioxygenase (KanJ)-an enzyme with a broad range of substrates

Kanamycin A is an aminoglycoside antibiotic isolated from Streptomyces kanamyceticus and used against a wide spectrum of bacteria, including Mycobacterium tuberculosis. Biosynthesis of kanamycin involves an oxidative deamination step catalyzed by kanamycin B dioxygenase (KanJ), thereby the C2' position of kanamycin B is transformed into a keto group upon release of ammonia. Here, we present for the first time, structural models of KanJ with several ligands, which along with the results of ITC binding assays and HPLC activity tests explain substrate specificity of the enzyme. The large size of the binding pocket suggests that KanJ can accept a broad range of substrates, which was confirmed by activity tests. Specificity of the enzyme with respect to its substrate is determined by the hydrogen bond interactions between the methylamino group of the antibiotic and highly conserved Asp134 and Cys150 as well as between hydroxyl groups of the substrate and Asn120 and Gln80. Upon antibiotic binding, the C terminus loop is significantly rearranged and Gln80 and Asn120, which are directly involved in substrate recognition, change their conformations. Based on reaction energy profiles obtained by density functional theory (DFT) simulations, we propose a mechanism of ketone formation involving the reactive Fe^IV  = O and proceeding either via OH rebound, which yields a hemiaminal intermediate or by abstraction of two hydrogen atoms, which leads to an imine species. At acidic pH, the latter involves a lower barrier than the OH rebound, whereas at basic pH, the barrier leading to an imine vanishes completely. DATABASES: Structural data are available in PDB database under the accession numbers: 6S0R, 6S0T, 6S0U, 6S0W, 6S0V, 6S0S. Diffraction images are available at the Integrated Resource for Reproducibility in Macromolecular Crystallography at http://proteindiffraction.org under DOIs: 10.18430/m36s0t, 10.18430/m36s0u, 10.18430/m36s0r, 10.18430/m36s0s, 10.18430/m36s0v, 10.18430/m36s0w. A data set collection of computational results is available in the Mendeley Data database under DOI: 10.17632/sbyzssjmp3.1 and in the ioChem-BD database under DOI: 10.19061/iochem-bd-4-18.

The inner centromere is a biomolecular condensate scaffolded by the chromosomal passenger complex

The inner centromere is a region on every mitotic chromosome that enables specific biochemical reactions that underlie properties, such as the maintenance of cohesion, the regulation of kinetochores and the assembly of specialized chromatin, that can resist microtubule pulling forces. The chromosomal passenger complex (CPC) is abundantly localized to the inner centromeres and it is unclear whether it is involved in non-kinase activities that contribute to the generation of these unique chromatin properties. We find that the borealin subunit of the CPC drives phase separation of the CPC in vitro at concentrations that are below those found on the inner centromere. We also provide strong evidence that the CPC exists in a phase-separated state at the inner centromere. CPC phase separation is required for its inner-centromere localization and function during mitosis. We suggest that the CPC combines phase separation, kinase and histone code-reading activities to enable the formation of a chromatin body with unique biochemical activities at the inner centromere.

Characterizing metal-binding sites in proteins with X-ray crystallography

Metals have crucial roles in many physiological, pathological, toxicological, pharmaceutical, and diagnostic processes. Proper handling of metal-containing macromolecule samples for structural studies is not trivial, and failure to handle them properly is often a source of irreproducibility caused by issues such as pH changes, incorporation of unexpected metals, or oxidization/reduction of the metal. This protocol outlines the guidelines and best practices for characterizing metal-binding sites in protein structures and alerts experimenters to potential pitfalls during the preparation and handling of metal-containing protein samples for X-ray crystallography studies. The protocol features strategies for controlling the sample pH and the metal oxidation state, recording X-ray fluorescence (XRF) spectra, and collecting diffraction data sets above and below the corresponding metal absorption edges. This protocol should allow experimenters to gather sufficient evidence to unambiguously determine the identity and location of the metal of interest, as well as to accurately characterize the coordinating ligands in the metal binding environment within the protein. Meticulous handling of metal-containing macromolecule samples as described in this protocol should enhance experimental reproducibility in biomedical sciences, especially in X-ray macromolecular crystallography. For most samples, the protocol can be completed within a period of 7-190 d, most of which (2-180 d) is devoted to growing the crystal. The protocol should be readily understandable to structural biologists, particularly protein crystallographers with an intermediate level of experience.

Crystal structure of thebaine 6-O-demethylase from the morphine biosynthesis pathway

Thebaine 6-O-demethylase (T6ODM) from Papaver somniferum (opium poppy), which belongs to the non-heme 2-oxoglutarate/Fe(II)-dependent dioxygenases (ODD) family, is a key enzyme in the morphine biosynthesis pathway. Initially, T6ODM was characterized as an enzyme catalyzing O-demethylation of thebaine to neopinone and oripavine to morphinone. However, the substrate range of T6ODM was recently expanded to a number of various benzylisoquinoline alkaloids. Here, we present crystal structures of T6ODM in complexes with 2-oxoglutarate (T6ODM:2OG, PDB: 5O9W) and succinate (T6ODM:SIN, PDB: 5O7Y). Both metal and 2OG binding sites display similarity to other proteins from the ODD family, but T6ODM is characterized by an exceptionally large substrate binding cavity, whose volume can partially explain the promiscuity of this enzyme. Moreover, the size of the cavity allows for binding of multiple molecules at once, posing a question about the substrate-driven specificity of the enzyme.

Reaction mechanism of sterol hydroxylation by steroid C25 dehydrogenase - Homology model, reactivity and isoenzymatic diversity

Steroid C25 dehydrogenase (S25DH) is a molybdenum-containing oxidoreductase isolated from the anaerobic Sterolibacterium denitrificans Chol-1S. S25DH is classified as 'EBDH-like' enzyme (EBDH, ethylbenzene dehydrogenase) and catalyzes the introduction of an OH group to the C25 atom of a sterol aliphatic side-chain. Due to its regioselectivity, S25DH is proposed as a catalyst in production of pharmaceuticals: calcifediol or 25-hydroxycholesterol. The aim of presented research was to obtain structural model of catalytic subunit α and investigate the reaction mechanism of the O₂-independent tertiary carbon atom activation. Based on homology modeling and theoretical calculations, a S25DH α subunit model was for the first time characterized and compared to other S25DH-like isoforms. The molecular dynamics simulations of the enzyme-substrate complexes revealed two stable binding modes of a substrate, which are stabilized predominantly by van der Waals forces in the hydrophobic substrate channel. However, H-bond interactions involving polar residues with C3=O/C3-OH in the steroid ring appear to be responsible for positioning the substrate. These results may explain the experimental kinetic results which showed that 3-ketosterols are hydroxylated 5-10-fold faster than 3-hydroxysterols. The reaction mechanism was studied using QM:MM and QM-only cluster models. The postulated mechanism involves homolytic CH cleavage by the MoO ligand, giving rise to a radical intermediate with product obtained in an OH rebound process. The hypothesis was supported by kinetic isotopic effect (KIE) experiments involving 25,26,26,26-[²H]-cholesterol (4.5) and the theoretically predicted intrinsic KIE (7.0-7.2). Finally, we have demonstrated that the recombinant S25DH-like isoform catalyzes the same reaction as S25DH.

Optimization of overexpression of a chaperone protein of steroid C25 dehydrogenase for biochemical and biophysical characterization

Molybdenum is an essential nutrient for metabolism in plant, bacteria, and animals. Molybdoenzymes are involved in nitrogen assimilation and oxidoreductive detoxification, and bioconversion reactions of environmental, industrial, and pharmaceutical interest. Molybdoenzymes contain a molybdenum cofactor (Moco), which is a pyranopterin heterocyclic compound that binds a molybdenum atom via a dithiolene group. Because Moco is a large and complex compound deeply buried within the protein, molybdoenzymes are accompanied by private chaperone proteins responsible for the cofactor's insertion into the enzyme and the enzyme's maturation. An efficient recombinant expression and purification of both Moco-free and Moco-containing molybdoenzymes and their chaperones is of paramount importance for fundamental and applied research related to molybdoenzymes. In this work, we focused on a D1 protein annotated as a chaperone of steroid C25 dehydrogenase (S25DH) from Sterolibacterium denitrificans Chol-1S. The D1 protein is presumably involved in the maturation of S25DH engaged in oxygen-independent oxidation of sterols. As this chaperone is thought to be a crucial element that ensures the insertion of Moco into the enzyme and consequently, proper folding of S25DH optimization of the chaperon's expression is the first step toward the development of recombinant expression and purification methods for S25DH. We have identified common E. coli strains and conditions for both expression and purification that allow us to selectively produce Moco-containing and Moco-free chaperones. We have also characterized the Moco-containing chaperone by EXAFS and HPLC analysis and identified conditions that stabilize both forms of the protein. The protocols presented here are efficient and result in protein quantities sufficient for biochemical studies.

Regioselective hydroxylation of cholecalciferol, cholesterol and other sterol derivatives by steroid C25 dehydrogenase

Steroid C25 dehydrogenase (S25DH) from Sterolibacterium denitrificans Chol-1S is a molybdenum oxidoreductase belonging to the so-called ethylbenzene dehydrogenase (EBDH)-like subclass of DMSO reductases capable of the regioselective hydroxylation of cholesterol or cholecalciferol to 25-hydroxy products. Both products are important biologically active molecules: 25-hydroxycholesterol is responsible for a complex regulatory function in the immunological system, while 25-hydroxycholecalciferol (calcifediol) is the activated form of vitamin D₃ used in the treatment of rickets and other calcium disorders. Studies revealed that the optimal enzymatic synthesis proceeds in fed-batch reactors under anaerobic conditions, with 6-9 % (w/v) 2-hydroxypropyl-β-cyclodextrin as a solubilizer and 1.25-5 % (v/v) 2-methoxyethanol as an organic co-solvent, both adjusted to the substrate type, and 8-15 mM K₃[Fe(CN)₆] as an electron acceptor. Such thorough optimization of the reaction conditions resulted in high product concentrations: 0.8 g/L for 25-hydroxycholesterol, 1.4 g/L for calcifediol and 2.2 g/L for 25-hydroxy-3-ketosterols. Although the purification protocol yields approximately 2.3 mg of pure S25DH from 30 g of wet cell mass (specific activity of 14 nmol min^-1 mg^-1), the non-purified crude extract or enzyme preparation can be readily used for the regioselective hydroxylation of both cholesterol and cholecalciferol. On the other hand, pure S25DH can be efficiently immobilized either on powder or a monolithic silica support functionalized with an organic linker providing NH₂ groups for enzyme covalent binding. Although such immobilization reduced the enzyme initial activity more than twofold it extended S25DH catalytic lifetime under working conditions at least 3.5 times.

Protein purification and crystallization artifacts: The tale usually not told

The misidentification of a protein sample, or contamination of a sample with the wrong protein, may be a potential reason for the non-reproducibility of experiments. This problem may occur in the process of heterologous overexpression and purification of recombinant proteins, as well as purification of proteins from natural sources. If the contaminated or misidentified sample is used for crystallization, in many cases the problem may not be detected until structures are determined. In the case of functional studies, the problem may not be detected for years. Here several procedures that can be successfully used for the identification of crystallized protein contaminants, including: (i) a lattice parameter search against known structures, (ii) sequence or fold identification from partially built models, and (iii) molecular replacement with common contaminants as search templates have been presented. A list of common contaminant structures to be used as alternative search models was provided. These methods were used to identify four cases of purification and crystallization artifacts. This report provides troubleshooting pointers for researchers facing difficulties in phasing or model building.

Molecular basis for phosphospecific recognition of histone H3 tails by Survivin paralogues at inner centromeres

The structure of hSurvivin bound to the histone H3 tail phosphorylated on Thr-3 was solved to determine how the CPC reads the histone code. Many eukaryotes have two Survivin paralogues. A major difference between them is that class A is pH sensitive in H3T3ph binding, whereas class B is relatively pH insensitive but has lower affinity for H3T3ph.

Survivin, a subunit of the chromosome passenger complex (CPC), binds the N-terminal tail of histone H3, which is phosphorylated on T3 by Haspin kinase, and localizes the complex to the inner centromeres. We used x-ray crystallography to determine the residues of Survivin that are important in binding phosphomodified histone H3. Mutation of amino acids that interact with the histone N-terminus lowered in vitro tail binding affinity and reduced CPC recruitment to the inner centromere in cells, validating our solved structures. Phylogenetic analysis shows that nonmammalian vertebrates have two Survivin paralogues, which we name class A and B. A distinguishing feature of these paralogues is an H-to-R change in an amino acid that interacts with the histone T3 phosphate. The binding to histone tails of the human class A paralogue, which has a histidine at this position, is sensitive to changes around physiological pH, whereas Xenopus Survivin class B is less so. Our data demonstrate that Survivin paralogues have different characteristics of phosphospecific binding to threonine-3 of histone H3, providing new insight into the biology of the inner centromere.

Histone H3 Thr-3 phosphorylation by Haspin positions Aurora B at centromeres in mitosis

Aurora B is a component of the chromosomal passenger complex (CPC) required for correct spindle-kinetochore attachments during chromosome segregation and for cytokinesis. The chromatin factors that recruit the CPC to centromeres are unknown, however. Here we show that phosphorylation of histone H3 threonine 3 (H3T3ph) by Haspin is necessary for CPC accumulation at centromeres and that the CPC subunit Survivin binds directly to H3T3ph. A nonbinding Survivin-D70A/D71A mutant does not support centromeric CPC concentration, and both Haspin depletion and Survivin-D70A/D71A mutation diminish centromere localization of the kinesin MCAK and the mitotic checkpoint response to taxol. Survivin-D70A/D71A mutation and microinjection of H3T3ph-specific antibody both compromise centromeric Aurora B functions but do not prevent cytokinesis. Therefore, H3T3ph generated by Haspin positions the CPC at centromeres to regulate selected targets of Aurora B during mitosis.