Molecular anatomy of the receptor binding module of a bacteriophage long tail fiber.

Tailed bacteriophages (phages) are one of the most abundant life forms on Earth. They encode highly efficient molecular machines to infect bacteria, but the initial interactions between a phage and a bacterium that then lead to irreversible virus attachment and infection are poorly understood. This information is critically needed to engineer machines with novel host specificities in order to combat antibiotic resistance, a major threat to global health today. The tailed phage T4 encodes a specialized device for this purpose, the long tail fiber (LTF), which allows the virus to move on the bacterial surface and find a suitable site for infection. Consequently, the infection efficiency of phage T4 is one of the highest, reaching the theoretical value of 1. Although the atomic structure of the tip of the LTF has been determined, its functional architecture and how interactions with two structurally very different Escherichia coli receptor molecules, lipopolysaccharide (LPS) and outer membrane protein C (OmpC), contribute to virus movement remained unknown. Here, by developing direct receptor binding assays, extensive mutational and biochemical analyses, and structural modeling, we discovered that the ball-shaped tip of the LTF, a trimer of gene product 37, consists of three sets of symmetrically alternating binding sites for LPS and/or OmpC. Our studies implicate reversible and dynamic interactions between these sites and the receptors. We speculate that the LTF might function as a "molecular pivot" allowing the virus to "walk" on the bacterium by adjusting the angle or position of interaction of the six LTFs attached to the six-fold symmetric baseplate.

Self-Immolation of a Bacterial Dehydratase Enzyme by its Epoxide Product.

Disabling the bacterial capacity to cause infection is an innovative approach that has attracted significant attention to fight against superbugs. A relevant target for anti-virulence drug discovery is the type I dehydroquinase (DHQ1) enzyme. It was shown that the 2-hydroxyethylammonium derivative 3 has in vitro activity since it causes the covalent modification of the catalytic lysine residue of DHQ1. As this compound does not bear reactive electrophilic centers, how the chemical modification occurs is intriguing. We report here an integrated approach, which involves biochemical studies, X-ray crystallography and computational studies on the reaction path using combined quantum mechanics/molecular mechanics Umbrella Sampling Molecular Dynamics, that evidences that DHQ1 catalyzes its self-immolation by transforming the unreactive 2-hydroxyethylammonium group in 3 into an epoxide that triggers the lysine covalent modification. This finding might open opportunities for the design of lysine-targeted irreversible inhibitors bearing a 2-hydroxyethylammonium moiety as an epoxide proform, which to our knowledge has not been reported previously.

Contractile injection systems of bacteriophages and related systems.

Contractile tail bacteriophages, or myobacteriophages, use a sophisticated biomolecular structure to inject their genome into the bacterial host cell. This structure consists of a contractile sheath enveloping a rigid tube that is sharpened by a spike-shaped protein complex at its tip. The spike complex forms the centerpiece of a baseplate complex that terminates the sheath and the tube. The baseplate anchors the tail to the target cell membrane with the help of fibrous proteins emanating from it and triggers contraction of the sheath. The contracting sheath drives the tube with its spiky tip through the target cell membrane. Subsequently, the bacteriophage genome is injected through the tube. The structural transformation of the bacteriophage T4 baseplate upon binding to the host cell has been recently described in near-atomic detail. In this review we discuss structural elements and features of this mechanism that are likely to be conserved in all contractile injection systems (systems evolutionary and structurally related to contractile bacteriophage tails). These include the type VI secretion system (T6SS), which is used by bacteria to transfer effectors into other bacteria and into eukaryotic cells, and tailocins, a large family of contractile bacteriophage tail-like compounds that includes the P. aeruginosa R-type pyocins.

Target highlights from the first post-PSI CASP experiment (CASP12, May-August 2016).

The functional and biological significance of the selected CASP12 targets are described by the authors of the structures. The crystallographers discuss the most interesting structural features of the target proteins and assess whether these features were correctly reproduced in the predictions submitted to the CASP12 experiment.

Structure and N-acetylglucosamine binding of the distal domain of mouse adenovirus 2 fibre.

Murine adenovirus 2 (MAdV-2) infects cells of the mouse gastrointestinal tract. Like human adenoviruses, it is a member of the genus Mastadenovirus, family Adenoviridae. The MAdV-2 genome has a single fibre gene that expresses a 787 residue-long protein. Through analogy to other adenovirus fibre proteins, it is expected that the carboxy-terminal virus-distal head domain of the fibre is responsible for binding to the host cell, although the natural receptor is unknown. The putative head domain has little sequence identity to adenovirus fibres of known structure. In this report, we present high-resolution crystal structures of the carboxy-terminal part of the MAdV-2 fibre. The structures reveal a domain with the typical adenovirus fibre head topology and a domain containing two triple ß-spiral repeats of the shaft domain. Through glycan microarray profiling, saturation transfer difference nuclear magnetic resonance spectroscopy, isothermal titration calorimetry and site-directed mutagenesis, we show that the fibre specifically binds to the monosaccharide N-acetylglucosamine (GlcNAc). The crystal structure of the complex reveals that GlcNAc binds between the AB and CD loops at the top of each of the three monomers of the MAdV-2 fibre head. However, infection competition assays show that soluble GlcNAc monosaccharide and natural GlcNAc-containing polymers do not inhibit infection by MAdV-2. Furthermore, site-directed mutation of the GlcNAc-binding residues does not prevent the inhibition of infection by soluble fibre protein. On the other hand, we show that the MAdV-2 fibre protein binds GlcNAc-containing mucin glycans, which suggests that the MAdV-2 fibre protein may play a role in viral mucin penetration in the mouse gut.

The so far farthest reaches of the double jelly roll capsid protein fold.

BACKGROUND: During the last two decades, structural biology analyses have shown that viruses infecting hosts far apart in evolution share similar architectural features, prompting a new virus classification based on structural lineages. Until recently, only a few prokaryotic viruses had been described for one of the lineages, whose main characteristic is a capsid protein with a perpendicular double jelly roll. MAIN BODY: Metagenomics analyses are showing that the variety of prokaryotic viruses encoding double jelly roll capsid proteins is much larger than previously thought. The newly discovered viruses have novel genome organisations with interesting implications for virus structure, function and evolution. There are also indications of their having a significant ecological impact. CONCLUSION: Viruses with double jelly roll capsid proteins that infect prokaryotic hosts form a large part of the virosphere that had so far gone unnoticed. Their discovery by metagenomics is only a first step towards many more exciting findings. Work needs to be invested in isolating these viruses and their hosts, characterizing the structure and function of the proteins their genomes encode, and eventually access the wealth of biological information they may hold.

Conformational changes leading to T7 DNA delivery upon interaction with the bacterial receptor.

The majority of bacteriophages protect their genetic material by packaging the nucleic acid in concentric layers to an almost crystalline concentration inside protein shells (capsid). This highly condensed genome also has to be efficiently injected into the host bacterium in a process named ejection. Most phages use a specialized complex (often a tail) to deliver the genome without disrupting cell integrity. Bacteriophage T7 belongs to the Podoviridae family and has a short, non-contractile tail formed by a tubular structure surrounded by fibers. Here we characterize the kinetics and structure of bacteriophage T7 DNA delivery process. We show that T7 recognizes lipopolysaccharides (LPS) from Escherichia coli rough strains through the fibers. Rough LPS acts as the main phage receptor and drives DNA ejection in vitro. The structural characterization of the phage tail after ejection using cryo-electron microscopy (cryo-EM) and single particle reconstruction methods revealed the major conformational changes needed for DNA delivery at low resolution. Interaction with the receptor causes fiber tilting and opening of the internal tail channel by untwisting the nozzle domain, allowing release of DNA and probably of the internal head proteins.

Some of the most interesting CASP11 targets through the eyes of their authors.

The Critical Assessment of protein Structure Prediction (CASP) experiment would not have been possible without the prediction targets provided by the experimental structural biology community. In this article, selected crystallographers providing targets for the CASP11 experiment discuss the functional and biological significance of the target proteins, highlight their most interesting structural features, and assess whether these features were correctly reproduced in the predictions submitted to CASP11. Proteins 2016; 84(Suppl 1):34-50. © 2015 The Authors. Proteins: Structure, Function, and Bioinformatics Published by Wiley Periodicals, Inc.

Crystal structure of raptor adenovirus 1 fibre head and role of the beta-hairpin in siadenovirus fibre head domains.

BACKGROUND: Most adenoviruses recognize their host cells via an interaction of their fibre head domains with a primary receptor. The structural framework of adenovirus fibre heads is conserved between the different adenovirus genera for which crystal structures have been determined (Mastadenovirus, Aviadenovirus, Atadenovirus and Siadenovirus), but genus-specific differences have also been observed. The only known siadenovirus fibre head structure, that of turkey adenovirus 3 (TAdV-3), revealed a twisted beta-sandwich resembling the reovirus fibre head architecture more than that of other adenovirus fibre heads, plus a unique beta-hairpin embracing a neighbouring monomer. The TAdV-3 fibre head was shown to bind sialyllactose. METHODS: Raptor adenovirus 1 (RAdV-1) fibre head was expressed, crystallized and its structure was solved and refined at 1.5 Å resolution. The structure could be solved by molecular replacement using the TAdV-3 fibre head structure as a search model, despite them sharing a sequence identity of only 19 %. Versions of both the RAdV-1 and TAdV-3 fibre heads with their beta-hairpin arm deleted were prepared and their stabilities were compared with the non-mutated proteins by a thermal unfolding assay. RESULTS: The structure of the RAdV-1 fibre head contains the same twisted ABCJ-GHID beta-sandwich and beta-hairpin arm as the TAdV-3 fibre head. However, while the predicted electro-potential surface charge of the TAdV-3 fibre head is mainly positive, the RAdV-1 fibre head shows positively and negatively charged patches and does not appear to bind sialyllactose. Deletion of the beta-hairpin arm does not affect the structure of the raptor adenovirus 1 fibre head and only affects the stability of the RAdV-1 and TAdV-3 fibre heads slightly. CONCLUSIONS: The high-resolution structure of RAdV-1 fibre head is the second known structure of a siadenovirus fibre head domain. The structure shows that the siadenovirus fibre head structure is conserved, but differences in the predicted surface charge suggest that RAdV-1 uses a different natural receptor for cell attachment than TAdV-3. Deletion of the beta-hairpin arm shows little impact on the structure and stability of the siadenovirus fibre heads.

Fasciola hepatica calcium-binding protein FhCaBP2: structure of the dynein light chain-like domain.

The common liver fluke Fasciola hepatica causes an increasing burden on human and animal health, partly because of the spread of drug-resistant isolates. As a consequence, there is considerable interest in developing new drugs to combat liver fluke infections. A group of potential targets is a family of calcium-binding proteins which combine an N-terminal domain with two EF-hand motifs and a C-terminal domain with predicted similarity to dynein light chains (DLC-like domain). The function of these proteins is unknown, although in several species, they have been localised to the tegument, an important structure at the host-parasite interface. Here, we report the X-ray crystal structure of the DLC-like domain of F. hepatica calcium-binding protein 2 (FhCaBP2), solved using single-wavelength anomalous diffraction and refined at 2.3 Å resolution in two different crystal forms. The FhCaBP2 DLC-like domain has a structure similar to other DLC domains, with an anti-parallel ß-sheet packed against an α-helical hairpin. Like other DLC domains, it dimerises through its ß2-strand, which extends in an arch and forms the fifth strand in an extended ß-sheet of the other monomer. The structure provides molecular details of the dimerisation of FhCaBP2, the first example from this family of parasite proteins.

The Eighth Central European Conference "Chemistry towards Biology": Snapshot.

The Eighth Central European Conference "Chemistry towards Biology" was held in Brno, Czech Republic, on August 28-September 1, 2016 to bring together experts in biology, chemistry and design of bioactive compounds; promote the exchange of scientific results, methods and ideas; and encourage cooperation between researchers from all over the world. The topics of the conference covered "Chemistry towards Biology", meaning that the event welcomed chemists working on biology-related problems, biologists using chemical methods, and students and other researchers of the respective areas that fall within the common scope of chemistry and biology. The authors of this manuscript are plenary speakers and other participants of the symposium and members of their research teams. The following summary highlights the major points/topics of the meeting.

Chemical Modification of a Dehydratase Enzyme Involved in Bacterial Virulence by an Ammonium Derivative: Evidence of its Active Site Covalent Adduct.

The first example of an ammonium derivative that causes a specific modification of the active site of type I dehydroquinase (DHQ1), a dehydratase enzyme that is a promising target for antivirulence drug discovery, is described. The resolution at 1.35 Å of the crystal structure of DHQ1 from Salmonella typhi chemically modified by this ammonium derivative revealed that the ligand is covalently attached to the essential Lys170 through the formation of an amine. The detection by mass spectroscopy of the reaction intermediates, in conjunction with the results of molecular dynamics simulations, allowed us to explain the inhibition mechanism and the experimentally observed differences between S. typhi and Staphylococcus aureus enzymes. The results presented here reveal that the replacement of Phe225 in St-DHQ1 by Tyr214 in Sa-DHQ1 and its hydrogen bonding interaction with the conserved water molecule observed in several crystal structures protects the amino adduct against further dehydration/aromatization reactions. In contrast, for the St-DHQ1 enzyme, the carboxylate group of Asp114, with the assistance of this water molecule, would trigger the formation of a Schiff base that can undergo further dehydration reactions until full aromatization of the cyclohexane ring is achieved. Moreover, in vitro antivirulence studies showed that the reported compound is able to reduce the ability of Salmonella Enteritidis to kill A459 respiratory cells. These studies have identified a good scaffold for the design of irreversible inhibitors that can be used as drugs and has opened up new opportunities for the development of novel antivirulence agents by targeting the DHQ1 enzyme.

Molecular characterization of a lizard adenovirus reveals the first atadenovirus with two fiber genes and the first adenovirus with either one short or three long fibers per penton.

UNLABELLED: Although adenoviruses (AdVs) have been found in a wide variety of reptiles, including numerous squamate species, turtles, and crocodiles, the number of reptilian adenovirus isolates is still scarce. The only fully sequenced reptilian adenovirus, snake adenovirus 1 (SnAdV-1), belongs to the Atadenovirus genus. Recently, two new atadenoviruses were isolated from a captive Gila monster (Heloderma suspectum) and Mexican beaded lizards (Heloderma horridum). Here we report the full genomic and proteomic characterization of the latter, designated lizard adenovirus 2 (LAdV-2). The double-stranded DNA (dsDNA) genome of LAdV-2 is 32,965 bp long, with an average G+C content of 44.16%. The overall arrangement and gene content of the LAdV-2 genome were largely concordant with those in other atadenoviruses, except for four novel open reading frames (ORFs) at the right end of the genome. Phylogeny reconstructions and plesiomorphic traits shared with SnAdV-1 further supported the assignment of LAdV-2 to the Atadenovirus genus. Surprisingly, two fiber genes were found for the first time in an atadenovirus. After optimizing the production of LAdV-2 in cell culture, we determined the protein compositions of the virions. The two fiber genes produce two fiber proteins of different sizes that are incorporated into the viral particles. Interestingly, the two different fiber proteins assemble as either one short or three long fiber projections per vertex. Stoichiometry estimations indicate that the long fiber triplet is present at only one or two vertices per virion. Neither triple fibers nor a mixed number of fibers per vertex had previously been reported for adenoviruses or any other virus. IMPORTANCE: Here we show that a lizard adenovirus, LAdV-2, has a penton architecture never observed before. LAdV-2 expresses two fiber proteins-one short and one long. In the virion, most vertices have one short fiber, but a few of them have three long fibers attached to the same penton base. This observation raises new intriguing questions on virus structure. How can the triple fiber attach to a pentameric vertex? What determines the number and location of each vertex type in the icosahedral particle? Since fibers are responsible for primary attachment to the host, this novel architecture also suggests a novel mode of cell entry for LAdV-2. Adenoviruses have a recognized potential in nanobiomedicine, but only a few of the more than 200 types found so far in nature have been characterized in detail. Exploring the taxonomic wealth of adenoviruses should improve our chances to successfully use them as therapeutic tools.

Crystal structure of the fibre head domain of bovine adenovirus 4, a ruminant atadenovirus.

BACKGROUND: In adenoviruses, primary host cell recognition is generally performed by the head domains of their homo-trimeric fibre proteins. This first interaction is reversible. A secondary, irreversible interaction subsequently takes place via other adenovirus capsid proteins and leads to a productive infection. Although many fibre head structures are known for human mastadenoviruses, not many animal adenovirus fibre head structures have been determined, especially not from those belonging to adenovirus genera other than Mastadenovirus. METHODS: We constructed an expression vector for the fibre head domain from a ruminant atadenovirus, bovine adenovirus 4 (BAdV-4), consisting of amino acids 414-535, expressed the protein in Escherichia coli, purified it by metal affinity and cation exchange chromatography and crystallized it. The structure was solved using single isomorphous replacement plus anomalous dispersion of a mercury derivative and refined against native data that extended to 1.2 Å resolution. RESULTS: Like in other adenoviruses, the BAdV-4 fibre head monomer contains a beta-sandwich consisting of ABCJ and GHID sheets. The topology is identical to the fibre head of the other studied atadenovirus, snake adenovirus 1 (SnAdV-1), including the alpha-helix in the DG-loop, despite of them having a sequence identity of only 15 %. There are also differences which may have implications for ligand binding. Beta-strands G and H are longer and differences in several surface-loops and surface charge are observed. CONCLUSIONS: Chimeric adenovirus fibres have been used to retarget adenovirus-based anti-cancer and gene therapy vectors. Ovine adenovirus 7 (OAdV-7), another ruminant atadenovirus, is intensively tested as a basis for such a vector. Here, we present the high-resolution atomic structure of the BAdV-4 fibre head domain, the second atadenovirus fibre head structure known and the first of an atadenovirus that infects a mammalian host. Future research should focus on the receptor-binding properties of these fibre head domains.

Irreversible covalent modification of type I dehydroquinase with a stable Schiff base.

The irreversible inhibition of type I dehydroquinase (DHQ1), the third enzyme of the shikimic acid pathway, is investigated by structural, biochemical and computational studies. Two epoxides, which are mimetics of the natural substrate, were designed as irreversible inhibitors of the DHQ1 enzyme and to study the binding requirements of the linkage to the enzyme. The epoxide with the S configuration caused the covalent modification of the protein whereas no reaction was obtained with its epimer. The first crystal structure of DHQ1 from Salmonella typhi covalently modified by the S epoxide, which is reported at 1.4 Å, revealed that the modified ligand is surprisingly covalently attached to the essential Lys170 by the formation of a stable Schiff base. The experimental and molecular dynamics simulation studies reported here highlight the huge importance of the conformation of the C3 carbon of the ligand for covalent linkage to this type of aldolase I enzyme, revealed the key role played by the essential His143 as a Lewis acid in this process and show the need for a neatly closed active site for catalysis.

Insights into substrate binding and catalysis in bacterial type I dehydroquinase.

Structural, biochemical and computational studies to study substrate binding and the role of the conserved residues of the DHQ1 (type I dehydroquinase) enzyme active site are reported in the present paper. The crystal structure of DHQ1 from Salmonella typhi in complex with (2R)-2-methyl-3-dehydroquinic acid, a substrate analogue, was solved at 1.5 Å. The present study reveals a previously unknown key role for conserved Glu46, Phe145 and Met205 and Gln236, Pro234 and Ala233 residues, with the latter three being located in the flexible substrate-covering loop. Gln236 was shown to be responsible for the folding of this loop and for the dramatic reduction of its flexibility, which triggers active site closure. Glu46 was found to be key in bringing the substrate close to the lysine/histidine catalytic pocket to initiate catalysis. The present study could be useful in the rational design of inhibitors of this challenging and recognized target for the development of novel herbicides and antimicrobial agents.

Structure of the receptor-binding carboxy-terminal domain of bacteriophage T7 tail fibers.

The six bacteriophage T7 tail fibers, homo-trimers of gene product 17, are thought to be responsible for the first specific, albeit reversible, attachment to Escherichia coli lipopolysaccharide. The protein trimer forms kinked fibers comprised of an amino-terminal tail-attachment domain, a slender shaft, and a carboxyl-terminal domain composed of several nodules. Previously, we expressed, purified, and crystallized a carboxyl-terminal fragment comprising residues 371-553. Here, we report the structure of this protein trimer, solved using anomalous diffraction and refined at 2 Å resolution. Amino acids 371-447 form a tapered pyramid with a triangular cross-section composed of interlocked ß-sheets from each of the three chains. The triangular pyramid domain has three α-helices at its narrow end, which are connected to a carboxyl-terminal three-blade ß-propeller tip domain by flexible loops. The monomers of this tip domain each contain an eight-stranded ß-sandwich. The exact topology of the ß-sandwich fold is novel, but similar to that of knob domains of other viral fibers and the phage Sf6 needle. Several host-range change mutants have been mapped to loops located on the top of this tip domain, suggesting that this surface of the tip domain interacts with receptors on the cell surface.

Structural characterization of the bacteriophage T7 tail machinery.

Most bacterial viruses need a specialized machinery, called "tail," to inject their genomes inside the bacterial cytoplasm without disrupting the cellular integrity. Bacteriophage T7 is a well characterized member of the Podoviridae family infecting Escherichia coli, and it has a short noncontractile tail that assembles sequentially on the viral head after DNA packaging. The T7 tail is a complex of around 2.7 MDa composed of at least four proteins as follows: the connector (gene product 8, gp8), the tail tubular proteins gp11 and gp12, and the fibers (gp17). Using cryo-electron microscopy and single particle image reconstruction techniques, we have determined the precise topology of the tail proteins by comparing the structure of the T7 tail extracted from viruses and a complex formed by recombinant gp8, gp11, and gp12 proteins. Furthermore, the order of assembly of the structural components within the complex was deduced from interaction assays with cloned and purified tail proteins. The existence of common folds among similar tail proteins allowed us to obtain pseudo-atomic threaded models of gp8 (connector) and gp11 (gatekeeper) proteins, which were docked into the corresponding cryo-EM volumes of the tail complex. This pseudo-atomic model of the connector-gatekeeper interaction revealed the existence of a common molecular architecture among viruses belonging to the three tailed bacteriophage families, strongly suggesting that a common molecular mechanism has been favored during evolution to coordinate the transition between DNA packaging and tail assembly.

Challenging the state of the art in protein structure prediction: Highlights of experimental target structures for the 10th Critical Assessment of Techniques for Protein Structure Prediction Experiment CASP10.

For the last two decades, CASP has assessed the state of the art in techniques for protein structure prediction and identified areas which required further development. CASP would not have been possible without the prediction targets provided by the experimental structural biology community. In the latest experiment, CASP10, more than 100 structures were suggested as prediction targets, some of which appeared to be extraordinarily difficult for modeling. In this article, authors of some of the most challenging targets discuss which specific scientific question motivated the experimental structure determination of the target protein, which structural features were especially interesting from a structural or functional perspective, and to what extent these features were correctly reproduced in the predictions submitted to CASP10. Specifically, the following targets will be presented: the acid-gated urea channel, a difficult to predict transmembrane protein from the important human pathogen Helicobacter pylori; the structure of human interleukin (IL)-34, a recently discovered helical cytokine; the structure of a functionally uncharacterized enzyme OrfY from Thermoproteus tenax formed by a gene duplication and a novel fold; an ORFan domain of mimivirus sulfhydryl oxidase R596; the fiber protein gene product 17 from bacteriophage T7; the bacteriophage CBA-120 tailspike protein; a virus coat protein from metagenomic samples of the marine environment; and finally, an unprecedented class of structure prediction targets based on engineered disulfide-rich small proteins.

Crystal structure of the lytic CHAP(K) domain of the endolysin LysK from Staphylococcus aureus bacteriophage K.

BACKGROUND: Bacteriophages encode endolysins to lyse their host cell and allow escape of their progeny. Endolysins are also active against Gram-positive bacteria when applied from the outside and are thus attractive anti-bacterial agents. LysK, an endolysin from staphylococcal phage K, contains an N-terminal cysteine-histidine dependent amido-hydrolase/peptidase domain (CHAP(K)), a central amidase domain and a C-terminal SH3b cell wall-binding domain. CHAP(K) cleaves bacterial peptidoglycan between the tetra-peptide stem and the penta-glycine bridge. METHODS: The CHAP(K) domain of LysK was crystallized and high-resolution diffraction data was collected both from a native protein crystal and a methylmercury chloride derivatized crystal. The anomalous signal contained in the derivative data allowed the location of heavy atom sites and phase determination. The resulting structures were completed, refined and analyzed. The presence of calcium and zinc ions in the structure was confirmed by X-ray fluorescence emission spectroscopy. Zymogram analysis was performed on the enzyme and selected site-directed mutants. RESULTS: The structure of CHAP(K) revealed a papain-like topology with a hydrophobic cleft, where the catalytic triad is located. Ordered buffer molecules present in this groove may mimic the peptidoglycan substrate. When compared to previously solved CHAP domains, CHAP(K) contains an additional lobe in its N-terminal domain, with a structural calcium ion, coordinated by residues Asp45, Asp47, Tyr49, His51 and Asp56. The presence of a zinc ion in the active site was also apparent, coordinated by the catalytic residue Cys54 and a possible substrate analogue. Site-directed mutagenesis was used to demonstrate that residues involved in calcium binding and of the proposed active site were important for enzyme activity. CONCLUSIONS: The high-resolution structure of the CHAP(K) domain of LysK was determined, suggesting the location of the active site, the substrate-binding groove and revealing the presence of a structurally important calcium ion. A zinc ion was found more loosely bound. Based on the structure, we propose a possible reaction mechanism. Future studies will be aimed at co-crystallizing CHAP(K) with substrate analogues and elucidating its role in the complete LysK protein. This, in turn, may lead to the design of site-directed mutants with altered activity or substrate specificity.