Crystal structure of Bacillus subtilis SPP1 phage gp23.1, a putative chaperone

SPP1 is a siphophage infecting the gram‐positive bacterium Bacillus subtilis. The SPP1 tail electron microscopy (EM) reconstruction revealed that it is mainly constituted by conserved structural proteins such as the major tail proteins (gp17.1), the tape measure protein (gp18), the Distal tail protein (Dit, gp19.1), and the Tail associated lysin (gp21). A group of five small genes (22–24.1) follows in the genome but it remains to be elucidated whether their protein products belong or not to the tail. Noteworthy, an unassigned EM density accounting for ～245 kDa is present at the distal end of the SPP1 tail‐tip. We report here the gp23.1 crystal structure at 1.6 Å resolution, a protein that lacks sequence identity to any known protein. We found that gp23.1 forms a hexamer both in the crystal lattice and in solution as revealed by light scattering measurements. The gp23.1 hexamer does not fit well in the unassigned SPP1 tail‐tip EM density and we hypothesize that this protein might act as a chaperone.     

Biogenesis of a Bacteriophage Long Non-Contractile Tail

Siphoviruses are main killers of bacteria. They use a long non-contractile tail to recognize the host cell and to deliver the genome from the viral capsid to the bacterial cytoplasm. Here, we define the molecular organization of the Bacillus subtilis bacteriophage SPP1 ~ 6.8 MDa tail and uncover its biogenesis mechanisms. A complex between gp21 and the tail distal protein (Dit) gp19.1 is assembled first to build the tail cap (gp19.1-gp21Nter) connected by a flexible hinge to the tail fiber (gp21Cter). The tip of the gp21Cter fiber is loosely associated to gp22. The cap provides a platform where tail tube proteins (TTPs) initiate polymerization around the tape measure protein gp18 (TMP), a reaction dependent on the non-structural tail assembly chaperones gp17.5 and gp17.5* (TACs). Gp17.5 is essential for stability of gp18 in the cell. Helical polymerization stops at a precise tube length followed by binding of proteins gp16.1 (TCP) and gp17 (THJP) to build the tail interface for attachment to the capsid portal system. This finding uncovers the function of the extensively conserved gp16.1-homologs in assembly of long tails. All SPP1 tail components, apart from gp22, share homology to conserved proteins whose coding genes’ synteny is broadly maintained in siphoviruses. They conceivably represent the minimal essential protein set necessary to build functional long tails. Proteins homologous to SPP1 tail building blocks feature a variety of add-on modules that diversify extensively the tail core structure, expanding its capability to bind host cells and to deliver the viral genome to the bacterial cytoplasm.     

Crystal Structure of Bacteriophage SPP1 Distal Tail Protein (gp19.1): A BASEPLATE HUB PARADIGM IN GRAM-POSITIVE INFECTING PHAGES*

Siphophage SPP1 infects the Gram-positive bacterium Bacillus subtilis using its long non-contractile tail and tail-tip. Electron microscopy (EM) previously allowed a low resolution assignment of most orf products belonging to these regions. We report here the structure of the SPP1 distal tail protein (Dit, gp19.1). The combination of x-ray crystallography, EM, and light scattering established that Dit is a back-to-back dimer of hexamers. However, Dit fitting in the virion EM maps was only possible with a hexamer located between the tail-tube and the tail-tip. Structure comparison revealed high similarity between Dit and a central component of lactophage baseplates. Sequence similarity search expanded its relatedness to several phage proteins, suggesting that Dit is a docking platform for the tail adsorption apparatus in Siphoviridae infecting Gram-positive bacteria and that its architecture is a paradigm for these hub proteins. Dit structural similarity extends also to non-contractile and contractile phage tail proteins (gpV_N and XkdM) as well as to components of the bacterial type 6 secretion system, supporting an evolutionary connection between all these devices.     

Revisiting the host adhesion determinants of Streptococcus thermophilus siphophages

Available 3D structures of bacteriophage modules combined with predictive bioinformatic algorithms enabled the identification of adhesion modules in 57 siphophages infecting Streptococcus thermophilus (St). We identified several carbohydrate-binding modules (CBMs) in so-called evolved distal tail (Dit) and tail-associated lysozyme (Tal) proteins of St phage baseplates. We examined the open reading frame (ORF) downstream of the Tal-encoding ORF and uncovered the presence of a putative p2-like receptor-binding protein (RBP). A 21 Å resolution electron microscopy structure of the baseplate of cos-phage STP1 revealed the presence of six elongated electron densities, surrounding the core of the baseplate, that harbour the p2-like RBPs at their tip. To verify the functionality of these modules, we expressed GFP- or mCherry-coupled Tal and putative RBP CBMs and observed by fluorescence microscopy that both modules bind to their corresponding St host, the putative RBP CBM with higher affinity than the Tal-associated one. The large number of CBM functional domains in St phages suggests that they play a contributory role in the infection process, a feature that we previously described in lactococcal phages and beyond, possibly representing a universal feature of the siphophage host-recognition apparatus.     

Crystal structure of Bacillus subtilis SPP1 phage gp22 shares fold similarity with a domain of lactococcal phage p2 RBP

SPP1 is a siphophage infecting the gram‐positive bacterium Bacillus subtilis. It is constituted by an icosahedric head and a long non‐contractile tail formed by gene products (gp) 17–21. A group of 5 small genes (gp 22–24.1) follows in the genome those coding for the main tail components. However, the belonging of the corresponding gp to the tail or to other parts of the phage is not documented. Among these, gp22 lacks sequence identity to any known protein. We report here the gp22 structure solved by X‐ray crystallography at 2.35 Å resolution. We found that gp22 is a monomer in solution and possesses a significant structural similarity with lactococcal phage p2 ORF 18 N‐terminal “shoulder” domain.     

A shifty stop for a hairy tail

The tail apparatus of the bacteriophage SPP1 is an extraordinary approximately 1600-A-long molecular machine. The tail mediates attachment of the virus to the host surface receptor, as well-as ejection of the viral genome into the host. The distal tip of the tail binds the extracellular ectodomain of the Bacillus subtilis receptor YueB, while the tail tube provides a conduit to funnel the viral genome into the host. This process, which culminates with the ejection of the approximately 44 kb of viral DNA across the thick, cell envelope of the Gram-positive bacterial cell, takes place in a time scale of seconds to minutes and represents a remarkable example of biotransformation. In this issue of Molecular Microbiology, Auzat et al. provide compelling evidence that the two major structural proteins of the SPP1 tail, gp17.1 (approximately 19.1 kDa) and gp17.1* (approximately 28 kDa), share a common N-terminal sequence, and that gp17.1* is generated by a translational frameshift in the gene 17.1. The extra domain fused to gp17.1* is synthesized by a +1 programmed translational frameshift at the end of gene 17.1, which leads to the synthesis of approximately one gp17.1* for every three equivalents of gp17.1. This finding extends our current knowledge of translational frameshifts and provides a framework to understand how Siphoviridae phages like SPP1 have developed long-tail machines using only two major structural proteins.     

Crystal Structure of pb9, the Distal Tail Protein of Bacteriophage T5: a Conserved Structural Motif among All Siphophages

The tail of Caudovirales bacteriophages serves as an adsorption device, a host cell wall-perforating machine, and a genome delivery pathway. In Siphoviridae, the assembly of the long and flexible tail is a highly cooperative and regulated process that is initiated from the proteins forming the distal tail tip complex. In Gram-positive-bacterium-infecting siphophages, the distal tail (Dit) protein has been structurally characterized and is proposed to represent a baseplate hub docking structure. It is organized as a hexameric ring that connects the tail tube and the adsorption device. In this study, we report the characterization of pb9, a tail tip protein of Escherichia coli bacteriophage T5. By immunolocalization, we show that pb9 is located in the upper part of the cone of the T5 tail tip, at the end of the tail tube. The crystal structure of pb9 reveals a two-domain protein. Domain A exhibits remarkable structural similarity with the N-terminal domain of known Dit proteins, while domain B adopts an oligosaccharide/oligonucleotide-binding fold (OB-fold) that is not shared by these proteins. We thus propose that pb9 is the Dit protein of T5, making it the first Dit protein described for a Gram-negative-bacterium-infecting siphophage. Multiple sequence alignments suggest that pb9 is a paradigm for a large family of Dit proteins of siphophages infecting mostly Gram-negative hosts. The modular structure of the Dit protein maintains the basic building block that would be conserved among all siphophages, combining it with a more divergent domain that might serve specific host adhesion properties.     

Role of bacteriophage SPP1 tail spike protein gp21 on host cell receptor binding and trigger of phage DNA ejection

Bacteriophages recognize and bind specific receptors to infect suitable hosts. Bacteriophage SPP1 targets at least two receptors of the Bacillus subtilis cell envelope, the glucosylated wall teichoic acids and the membrane protein YueB. Here, we identify a key virion protein for YueB binding and for the trigger of DNA ejection. Extracts from B. subtilis-infected cells applied to a YueB affinity matrix led to preferential capturing of gp21 from SPP1. To assess the significance of this interaction, we isolated mutant phages specifically affected in YueB binding. The mutants exhibited a very low inactivation rate and a strong defect to eject DNA when challenged with YueB. The phenotype correlated with presence of a single amino acid substitution in the gp21 carboxyl terminus, defining a region involved in YueB binding. Immunoelectron microscopy located the gp21 N-terminus in the SPP1 cap and probably in the adjacent tail spike region whereas the gp21 C-terminus was mapped further down in the spike structure. Antibodies against this part of gp21 interfered with the interaction of YueB with SPP1 and triggered DNA ejection. The gp21 C-terminal region thus plays a central role in two early key events that commit the virus to deliver its genome into host cells.     

Structure and molecular assignment of lactococcal phage TP901-1 baseplate

P335 lactococcal phages infect the gram(+) bacterium Lactococcus lactis using a large multiprotein complex located at the distal part of the tail and termed baseplate (BP). The BP harbors the receptor-binding proteins (RBPs), which allow the specific recognition of saccharidic receptors localized on the host cell surface. We report here the electron microscopic structure of the phage TP901-1 wild-type BP as well as those of two mutants bppL (-) and bppU(-), lacking BppL (the RBPs) or both peripheral BP components (BppL and BppU), respectively. We also achieved an electron microscopic reconstruction of a partial BP complex, formed by BppU and BppL. This complex exhibits a tripod shape and is composed of nine BppLs and three BppUs. These structures, combined with light-scattering measurements, led us to propose that the TP901-1 BP harbors six tripods at its periphery, located around the central tube formed by ORF46 (Dit) hexamers, at its proximal end, and a ORF47 (Tal) trimer at its distal extremity. A total of 54 BppLs (18 RBPs) are thus available to mediate host anchoring with a large apparent avidity. TP901-1 BP exhibits an infection-ready conformation and differs strikingly from the lactococcal phage p2 BP, bearing only 6 RBPs, and which needs a conformational change to reach its activated state. The comparison of several Siphoviridae structures uncovers a close organization of their central BP core whereas striking differences occur at the periphery, leading to diverse mechanisms of host recognition.     

Structural characterization and assembly of the distal tail structure of the temperate lactococcal bacteriophage TP901-1

The tail structures of bacteriophages infecting gram-positive bacteria are largely unexplored, although the phage tail mediates the initial interaction with the host cell. The temperate Lactococcus lactis phage TP901-1 of the Siphoviridae family has a long noncontractile tail with a distal baseplate. In the present study, we investigated the distal tail structures and tail assembly of phage TP901-1 by introducing nonsense mutations into the late transcribed genes dit (orf46), tal(TP901-1) (orf47), bppU (orf48), bppL (orf49), and orf50. Transmission electron microscopy examination of mutant and wild-type TP901-1 phages showed that the baseplate consisted of two different disks and that a central tail fiber is protruding below the baseplate. Evaluation of the mutant tail morphologies with protein profiles and Western blots revealed that the upper and lower baseplate disks consist of the proteins BppU and BppL, respectively. Likewise, Dit and Tal(TP901-1) were shown to be structural tail proteins essential for tail formation, and Tal(TP901-1) was furthermore identified as the tail fiber protein by immunogold labeling experiments. Determination of infection efficiencies of the mutant phages showed that the baseplate is fundamental for host infection and the lower disk protein, BppL, is suggested to interact with the host receptor. In contrast, ORF50 was found to be nonessential for tail assembly and host infection. A model for TP901-1 tail assembly, in which the function of eight specific proteins is considered, is presented.     

Structural organisation of the head-to-tail interface of a bacterial virus

In tailed icosahedral bacteriophages the connection between the 5-fold symmetric environment of the portal vertex in the capsid and the 6-fold symmetric phage tail is formed by a complex interface structure. The current study provides the detailed analysis of the assembly and structural organisation of such an interface within a phage having a long tail. The region of the interface assembled as part of the viral capsid (connector) was purified from DNA-filled capsids of the Bacillus subtilis bacteriophage SPP1. It is composed of oligomers of gp6, the SPP1 portal protein, of gp15, and of gp16. The SPP1 connector structure is formed by a mushroom-like portal protein whose cap faces the interior of the viral capsid in intact virions, an annular structure below the stem of the mushroom, and a second narrower annulus that is in direct contact with the helical tail extremity. The layered arrangement correlates to the stacking of gp6, gp15, and gp16 on top of the tail. The gp16 ring is exposed to the virion outside. During SPP1 morphogenesis, gp6 participates in the procapsid assembly reaction, an early step in the assembly pathway, while gp15 and gp16 bind to the capsid portal vertex after viral chromosome encapsidation. gp16 is processed during or after tail attachment to the connector region. The portal protein gp6 has 12-fold cyclical symmetry in the connector structure, whereas assembly-na?ve gp6 exhibits 13-fold symmetry. We propose that it is the interaction of gp6 with other viral morphogenetic proteins that drives its assembly into the 12-mer state.     

The Collagen-like Protein gp12 Is a Temperature-dependent Reversible Binder of SPP1 Viral Capsids

Icosahedral capsids of viruses are lattices of defined geometry and homogeneous size. The (quasi-)equivalent organization of their protein building blocks provides, in numerous systems, the binding sites to assemble arrays of viral polypeptides organized with nanometer precision that protrude from the capsid surface. The capsid of bacterial virus (bacteriophage) SPP1 exposes, at its surface, the 6.6-kDa viral polypeptide gp12 that binds to the center of hexamers of the major capsid protein. Gp12 forms an elongated trimer with collagen-like properties. This is consistent with the fold of eight internal GXY repeats of gp12 to build a stable intersubunit triple helix in a prokaryotic setting. The trimer dissociates and unfolds at near physiological temperatures, as reported for eukaryotic collagen. Its structural organization is reacquired within seconds upon cooling. Interaction with the SPP1 capsid hexamers strongly stabilizes gp12, increasing its T_m to 54 °C. Above this temperature, gp12 dissociates from its binding sites and unfolds reversibly. Multivalent binding of gp12 trimers to the capsid is highly cooperative. The capsid lattice also provides a platform to assist folding and association of unfolded gp12 polypeptides. The original physicochemical properties of gp12 offer a thermoswitchable system for multivalent binding of the polypeptide to the SPP1 capsid 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.     

Direct Interaction of the Bacteriophage SPP1 Packaging ATPase with the Portal Protein

DNA packaging in tailed bacteriophages and other viruses requires assembly of a complex molecular machine at a specific vertex of the procapsid. This machine is composed of the portal protein that provides a tunnel for DNA entry, an ATPase that fuels DNA translocation (large terminase subunit), and most frequently, a small terminase subunit. Here we characterized the interaction between the terminase ATPase subunit of bacteriophage SPP1 (gp2) and the procapsid portal vertex. We found, by affinity pulldown assays with purified proteins, that gp2 interacts with the portal protein, gp6, independently of the terminase small subunit gp1, DNA, or ATP. The gp2-procapsid interaction via the portal protein depends on gp2 concentration and requires the presence of divalent cations. Competition experiments showed that isolated gp6 can only inhibit gp2-procapsid interactions and DNA packaging at gp6:procapsid molar ratios above 10-fold. Assays with gp6 carrying mutations in distinct regions of its structure that affect the portal-induced stimulation of ATPase and DNA packaging revealed that none of these mutations impedes gp2-gp6 binding. Our results demonstrate that the SPP1 packaging ATPase binds directly to the portal and that the interaction is stronger with the portal embedded in procapsids. Identification of mutations in gp6 that allow for assembly of the ATPase-portal complex but impair DNA packaging support an intricate cross-talk between the two proteins for activity of the DNA translocation motor.     

Structural Plasticity of the Protein Plug That Traps Newly Packaged Genomes in Podoviridae Virions

Bacterial viruses of the P22-like family encode a specialized tail needle essential for genome stabilization after DNA packaging and implicated in Gram-negative cell envelope penetration. The atomic structure of P22 tail needle (gp26) crystallized at acidic pH reveals a slender fiber containing an N-terminal “trimer of hairpins” tip. Although the length and composition of tail needles vary significantly in Podoviridae, unexpectedly, the amino acid sequence of the N-terminal tip is exceptionally conserved in more than 200 genomes of P22-like phages and prophages. In this paper, we used x-ray crystallography and EM to investigate the neutral pH structure of three tail needles from bacteriophage P22, HK620, and Sf6. In all cases, we found that the N-terminal tip is poorly structured, in stark contrast to the compact trimer of hairpins seen in gp26 crystallized at acidic pH. Hydrogen-deuterium exchange mass spectrometry, limited proteolysis, circular dichroism spectroscopy, and gel filtration chromatography revealed that the N-terminal tip is highly dynamic in solution and unlikely to adopt a stable trimeric conformation at physiological pH. This is supported by the cryo-EM reconstruction of P22 mature virion tail, where the density of gp26 N-terminal tip is incompatible with a trimer of hairpins. We propose the tail needle N-terminal tip exists in two conformations: a pre-ejection extended conformation, which seals the portal vertex after genome packaging, and a postejection trimer of hairpins, which forms upon its release from the virion. The conformational plasticity of the tail needle N-terminal tip is built in the amino acid sequence, explaining its extraordinary conservation in nature.     

A touch of glue to complete bacteriophage assembly: the tail‐to‐head joining protein (THJP) family

Bacteriophage SPP1 is a nanomachine built to infect the bacterium Bacillus subtilis. The phage particle is composed of an icosahedric capsid, which contains the viral DNA, and a long non‐contractile tail. Capsids and tails are produced in infected cells by two distinct morphogenetic pathways. Characterization of the suppressor‐sensitive mutant SPP1sus82 showed that it produces DNA‐filled capsids and tails but is unable to assemble complete virions. Its purified tails have a normal length but lack a narrow ring that tapers the tail end found at the tail‐to‐head interface. The mutant is defective in production of gp17. The gp17 ring is exposed in free tails competent for viral assembly but becomes shielded in the final virion structure. Recombinant gp17 is active in an in vitro assay to stick together capsids and tails present in extracts of SPP1sus82‐infected cells, leading to formation of infectious particles. Gp17 thus plays a fundamental role in the tail‐to‐head joining reaction, the ultimate step of virus particle assembly. This is the conserved function of gp17 and its structurally related proteins like lambda gpU. This family of proteins can also provide fidelity to termination of the tail tube elongation reaction in a subset of phages including coliphage lambda.     

Haloarchaeal myovirus φCh1 harbours a phase variation system for the production of protein variants with distinct cell surface adhesion specificities

The φCh1 myovirus, which infects the haloalkaliphilic archaeon Natrialba magadii, contains an invertible region that comprises the convergent open reading frames (ORFs) 34 and 36, which code for the putative tail fibre proteins gp34 and gp36 respectively. The inversion leads to an exchange of the C-termini of these proteins, thereby creating different types of tail fibres. Gene expression experiments revealed that only ORF34 is transcribed, indicating that φCh1 produces tail fibre proteins exclusively from this particular ORF. Only one of the two types of tail fibres encoded by ORF34 is able to bind to Nab. magadii in vitro. This is reflected by the observation that during the early phases of the infection cycle, the lysogenic strain L11 carries its invertible region exclusively in the orientation that produces that specific type of tail fibre. Obviously, Nab. magadii can only be infected by viruses carrying this particular type of tail fibre. By mutational analysis, the binding domain of gp34 was localized to the C-terminal part of the protein, particularly to a galactose-binding domain. The involvement of galactose residues in cell adhesion was supported by the observation that the addition of α-D-galactose to purified gp34 or whole virions prevented their attachment to Nab. magadii.     

Origin and function of the two major tail proteins of bacteriophage SPP1

The majority of bacteriophages have a long non-contractile tail (Siphoviridae) that serves as a conduit for viral DNA traffic from the phage capsid to the host cell at the beginning of infection. The 160-nm-long tail tube of Bacillus subtilis bacteriophage SPP1 is shown to be composed of two major tail proteins (MTPs), gp17.1 and gp17.1*, at a ratio of about 3:1. They share a common amino-terminus, but the latter species has approximately 10 kDa more than gp17.1. A CCC.UAA sequence with overlapping proline codons at the 3' end of gene 17.1 drives a programmed translational frameshift to another open reading frame. The recoding event generates gp17.1*. Phages carrying exclusively gp17.1 or gp17.1* are viable, but tails are structurally distinct. gp17.1 and the carboxyl-terminus of gp17.1* have a distinct evolutionary history correlating with different functions: the polypeptide sequence identical in the two proteins is responsible for assembly of the tail tube while the additional module of gp17.1* shields the structure exterior exposed to the environment. The carboxyl-terminal extension is an elaboration present in some tailed bacteriophages. Different extensions were found to combine in a mosaic fashion with the MTP essential module in a subset of Siphoviridae genomes.     

Highly conserved structural properties of the C-terminal tail of HIV-1 gp41 protein despite substantial sequence variation among diverse clades: implications for functions in viral replication

Although the HIV-1 Env gp120 and gp41 ectodomain have been extensively characterized in terms of structure and function, similar characterizations of the C-terminal tail (CTT) of HIV gp41 remain relatively limited and contradictory. The current study was designed to examine in detail CTT sequence conservation relative to gp120 and the gp41 ectodomain and to examine the conservation of predicted physicochemical and structural properties across a number of divergent HIV clades and groups. Results demonstrate that CTT sequences display intermediate levels of sequence evolution and diversity in comparison to the more diverse gp120 and the more conserved gp41 ectodomain. Despite the relatively high level of CTT sequence variation, the physicochemical properties of the lentivirus lytic peptide domains (LLPs) within the CTT are evidently highly conserved across clades/groups. Additionally, predictions using PEP-FOLD indicate a high level of structural similarity in the LLP regions that was confirmed by circular dichroism measurements of secondary structure of LLP peptides from clades B, C, and group O. Results demonstrate that LLP peptides adopt helical structure in the presence of SDS or trifluoroethanol but are predominantly unstructured in aqueous buffer. Thus, these data for the first time demonstrate strong conservations of characteristic CTT physicochemical and structural properties despite substantial sequence diversity, apparently indicating a delicate balance between evolutionary pressures and the conservation of CTT structure and associated functional roles in virus replication.     

Capsid structure and its stability at the late stages of bacteriophage SPP1 assembly

The structure of the bacteriophage SPP1 capsid was determined at subnanometer resolution by cryo-electron microscopy and single-particle analysis. The icosahedral capsid is composed of the major capsid protein gp13 and the auxiliary protein gp12, which are organized in a T=7 lattice. DNA is arranged in layers with a distance of ∼24.5 Å. gp12 forms spikes that are anchored at the center of gp13 hexamers. In a gp12-deficient mutant, the centers of hexamers are closed by loops of gp13 coming together to protect the SPP1 genome from the outside environment. The HK97-like fold was used to build a pseudoatomic model of gp13. Its structural organization remains unchanged upon tail binding and following DNA release. gp13 exhibits enhanced thermostability in the DNA-filled capsid. A remarkable convergence between the thermostability of the capsid and those of the other virion components was found, revealing that the overall architecture of the SPP1 infectious particle coevolved toward high robustness.