Crystallization of the C-terminal head domain of the fibre protein from a siadenovirus, turkey adenovirus 3

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 β-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.

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.

Structure and Sialyllactose Binding of the Carboxy-Terminal Head Domain of the Fibre from a Siadenovirus, Turkey Adenovirus 3

The virulent form of turkey adenovirus 3 (TAdV-3), also known as turkey hemorrhagic enteritis virus (THEV), is an economically important poultry pathogen, while the avirulent form is used as a vaccine. TAdV-3 belongs to the genus Siadenovirus. The carboxy-terminal region of its fibre does not have significant sequence similarity to any other adenovirus fibre heads of known structure. Two amino acid sequence differences between virulent and avirulent TAdV-3 map on the fibre head: where virulent TAdV-3 contains Ile354 and Thr376, avirulent TAdV-3 contains Met354 and Met376. We determined the crystal structures of the trimeric virulent and avirulent TAdV-3 fibre head domains at 2.2 Å resolution. Each monomer contains a beta-sandwich, which, surprisingly, resembles reovirus fibre head more than other adenovirus fibres, although the ABCJ-GHID topology is conserved in all. A beta-hairpin insertion in the C-strand of each trimer subunit embraces its neighbouring monomer. The avirulent and virulent TAdV-3 fibre heads are identical apart from the exact orientation of the beta-hairpin insertion. In vitro, sialyllactose was identified as a ligand by glycan microarray analysis, nuclear magnetic resonance spectroscopy, and crystallography. Its dissociation constant was measured to be in the mM range by isothermal titration calorimetry. The ligand binds to the side of the fibre head, involving amino acids Glu392, Thr419, Val420, Lys421, Asn422, and Gly423 binding to the sialic acid group. It binds slightly more strongly to the avirulent form. We propose that, in vivo, the TAdV-3 fibre may bind a sialic acid-containing cell surface component.

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.

Crystal structure of the fibre head domain of the Atadenovirus Snake Adenovirus 1

Adenoviruses are non-enveloped icosahedral viruses with trimeric fibre proteins protruding from their vertices. There are five known genera, from which only Mastadenoviruses have been widely studied. Apart from studying adenovirus as a biological model system and with a view to prevent or combat viral infection, there is a major interest in using adenovirus for vaccination, cancer therapy and gene therapy purposes. Adenoviruses from the Atadenovirus genus have been isolated from squamate reptile hosts, ruminants and birds and have a characteristic gene organization and capsid morphology. The carboxy-terminal virus-distal fibre head domains are likely responsible for primary receptor recognition. We determined the high-resolution crystal structure of the Snake Adenovirus 1 (SnAdV-1) fibre head using the multi-wavelength anomalous dispersion (MAD) method. Despite the absence of significant sequence homology, this Atadenovirus fibre head has the same beta-sandwich propeller topology as other adenovirus fibre heads. However, it is about half the size, mainly due to much shorter loops connecting the beta-strands. The detailed structure of the SnAdV-1 fibre head and other animal adenovirus fibre heads, together with the future identification of their natural receptors, may lead to the development of new strategies to target adenovirus vectors to cells of interest.

Crystallization of the C-terminal domain of the fibre protein from snake adenovirus 1, an atadenovirus

Adenovirus fibre proteins play an important role in determining viral tropism. The C-terminal domain of the fibre protein from snake adenovirus type 1, a member of the Atadenovirus genus, has been expressed, purified and crystallized. Crystals were obtained belonging to space groups P2(1)2(1)2(1) (two different forms), I2(1)3 and F23. The best of these diffracted synchrotron radiation to a resolution of 1.4 Å. As the protein lacks methionines or cysteines, site-directed mutagenesis was performed to change two leucine residues to methionines. Crystals of selenomethionine-derivatized crystals of the I2(1)3 form were also obtained and a multi-wavelength anomalous dispersion data set was collected.