Structure of the siphophage neck-Tail complex suggests that conserved tail tip proteins facilitate receptor binding and tail assembly.

Siphophages have a long, flexible, and noncontractile tail that connects to the capsid through a neck. The phage tail is essential for host cell recognition and virus-host cell interactions; moreover, it serves as a channel for genome delivery during infection. However, the in situ high-resolution structure of the neck-tail complex of siphophages remains unknown. Here, we present the structure of the siphophage lambda "wild type," the most widely used, laboratory-adapted fiberless mutant. The neck-tail complex comprises a channel formed by stacked 12-fold and hexameric rings and a 3-fold symmetrical tip. The interactions among DNA and a total of 246 tail protein molecules forming the tail and neck have been characterized. Structural comparisons of the tail tips, the most diversified region across the lambda and other long-tailed phages or tail-like machines, suggest that their tail tip contains conserved domains, which facilitate tail assembly, receptor binding, cell adsorption, and DNA retaining/releasing. These domains are distributed in different tail tip proteins in different phages or tail-like machines. The side tail fibers are not required for the phage particle to orient itself vertically to the surface of the host cell during attachment.

Structural Insights into the Chaperone-Assisted Assembly of a Simplified Tail Fiber of the Myocyanophage Pam3.

At the first step of phage infection, the receptor-binding proteins (RBPs) such as tail fibers are responsible for recognizing specific host surface receptors. The proper folding and assembly of tail fibers usually requires a chaperone encoded by the phage genome. Despite extensive studies on phage structures, the molecular mechanism of phage tail fiber assembly remains largely unknown. Here, using a minimal myocyanophage, termed Pam3, isolated from Lake Chaohu, we demonstrate that the chaperone gp25 forms a stable complex with the tail fiber gp24 at a stoichiometry of 3:3. The 3.1-Å cryo-electron microscopy structure of this complex revealed an elongated structure with the gp25 trimer embracing the distal moieties of gp24 trimer at the center. Each gp24 subunit consists of three domains: the N-terminal α-helical domain required for docking to the baseplate, the tumor necrosis factor (TNF)-like and glycine-rich domains responsible for recognizing the host receptor. Each gp25 subunit consists of two domains: a non-conserved N-terminal ß-sandwich domain that binds to the TNF-like and glycine-rich domains of the fiber, and a C-terminal α-helical domain that mediates trimerization/assembly of the fiber. Structural analysis enabled us to propose the assembly mechanism of phage tail fibers, in which the chaperone first protects the intertwined and repetitive distal moiety of each fiber subunit, further ensures the proper folding of these highly plastic structural elements, and eventually enables the formation of the trimeric fiber. These findings provide the structural basis for the design and engineering of phage fibers for biotechnological applications.

Structure of Escherichia coli O157:H7 bacteriophage CBA120 tailspike protein 4 baseplate anchor and tailspike assembly domains (TSP4-N).

Four tailspike proteins (TSP1-4) of Escherichia coli O157:H7 bacteriophage CBA120 enable infection of multiple hosts. They form a branched complex that attaches to the tail baseplate. Each TSP recognizes a different lipopolysaccharide on the membrane of a different bacterial host. The 335 N-terminal residues of TSP4 promote the assembly of the TSP complex and anchor it to the tail baseplate. The crystal structure of TSP4-N335 reveals a trimeric protein comprising four domains. The baseplate anchor domain (AD) contains an intertwined triple-stranded ß-helix. The ensuing XD1, XD2 and XD3 ß-sheet containing domains mediate the binding of TSP1-3 to TSP4. Each of the XD domains adopts the same fold as the respective XD domains of bacteriophage T4 gp10 baseplate protein, known to engage in protein-protein interactions via its XD2 and XD3 domains. The structural similarity suggests that XD2 and XD3 of TSP4 also function in protein-protein interactions. Analytical ultracentrifugation analyses of TSP4-N335 and of domain deletion proteins showed how TSP4-N335 promotes the formation of the TSP quaternary complex. TSP1 and TSP2 bind directly to TSP4 whereas TSP3 binding requires a pre-formed TSP4-N335:TSP2 complex. A 3-dimensional model of the bacteriophage CBA120 TSP complex has been developed based on the structural and ultracentrifuge information.

Identification of the tail assembly chaperone genes of T4-Like phages suggests a mechanism other than translational frameshifting for biogenesis of their encoded proteins.

Tape measure (TM) proteins are essential for the formation of long-tailed phages. TM protein assembly into tails requires the action of tail assembly chaperones (TACs). TACs (e.g. gpG and gpT of E. coli phage lambda) are usually produced in a short (TAC-N) and long form (TAC-NC) with the latter comprised of TAC-N with an additional C-terminal domain (TAC-C). TAC-NC is generally synthesized through a ribosomal frameshifting mechanism. TAC encoding genes have never been identified in the intensively studied Escherichia coli phage T4, or any related phages. Here, we have bioinformatically identified putative TAC encoding genes in diverse T4-like phage genomes. The frameshifting mechanism for producing TAC-NC appears to be conserved in several T4-like phage groups. However, the group including phage T4 itself likely employs a different strategy whereby TAC-N and TAC-NC are encoded by separate genes (26 and 51 in phage T4).

Structural Studies of the Phage G Tail Demonstrate an Atypical Tail Contraction.

Phage G is recognized as having a remarkably large genome and capsid size among isolated, propagated phages. Negative stain electron microscopy of the host-phage G interaction reveals tail sheaths that are contracted towards the distal tip and decoupled from the head-neck region. This is different from the typical myophage tail contraction, where the sheath contracts upward, while being linked to the head-neck region. Our cryo-EM structures of the non-contracted and contracted tail sheath show that: (1) The protein fold of the sheath protein is very similar to its counterpart in smaller, contractile phages such as T4 and phi812; (2) Phage G's sheath structure in the non-contracted and contracted states are similar to phage T4's sheath structure. Similarity to other myophages is confirmed by a comparison-based study of the tail sheath's helical symmetry, the sheath protein's evolutionary timetree, and the organization of genes involved in tail morphogenesis. Atypical phase G tail contraction could be due to a missing anchor point at the upper end of the tail sheath that allows the decoupling of the sheath from the head-neck region. Explaining the atypical tail contraction requires further investigation of the phage G sheath anchor points.

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.

Engineering the Modular Receptor-Binding Proteins of Klebsiella Phages Switches Their Capsule Serotype Specificity.

The high specificity of bacteriophages is driven by their receptor-binding proteins (RBPs). Many Klebsiella bacteriophages target the capsular exopolysaccharide as the receptor and encode RBPs with depolymerase activity. The modular structure of these RBPs with an N-terminal structural module to attach the RBP to the phage tail, and a C-terminal specificity module for exopolysaccharide degradation, supports horizontal transfer as a major evolutionary driver for Klebsiella phage RBPs. We mimicked this natural evolutionary process by the construction of modular RBP chimeras, exchanging N-terminal structural modules and C-terminal specificity modules. All chimeras strictly follow the capsular serotype specificity of the C-terminal module. Transplanting chimeras with a K11 N-terminal structural RBP module in a Klebsiella phage K11 scaffold results in a capsular serotype switch and corresponding host range modification of the synthetic phages, demonstrating that horizontal transfer of C-terminal specificity modules offers Klebsiella phages an evolutionary highway for rapid adaptation to new capsular serotypes.IMPORTANCE The antimicrobial resistance crisis has rekindled interest in bacteriophage therapy. Phages have been studied over a century as therapeutics to treat bacterial infections, but one of the biggest challenges for the use of phages in therapeutic interventions remains their high specificity. In particular, many Klebsiella phages have a narrow spectrum constrained by the high diversity of exopolysaccharide capsules that shield access to the cells. In this work, we have elaborated how Klebsiella phages deal with this high diversity by exchanging building blocks of their receptor-binding proteins.

A Tail Fiber Protein and a Receptor-Binding Protein Mediate ICP2 Bacteriophage Interactions with Vibrio cholerae OmpU.

ICP2 is a virulent bacteriophage (phage) that preys on Vibrio cholerae. ICP2 was first isolated from cholera patient stool samples. Some of these stools also contained ICP2-resistant isogenic V. cholerae strains harboring missense mutations in the trimeric outer membrane porin protein OmpU, identifying it as the ICP2 receptor. In this study, we identify the ICP2 proteins that mediate interactions with OmpU by selecting for ICP2 host range mutants within infant rabbits infected with a mixture of wild-type and OmpU mutant strains. ICP2 host range mutants that can now infect OmpU mutant strains have missense mutations in the putative tail fiber gene gp25 and the putative adhesin gene gp23. Using site-specific mutagenesis, we show that single or double mutations in gp25 are sufficient to generate the host range mutant phenotype. However, at least one additional mutation in gp23 is required for robust plaque formation on specific OmpU mutants. Mutations in gp23 alone were insufficient to produce a host range mutant phenotype. All ICP2 host range mutants retained the ability to form plaques on wild-type V. cholerae cells. The strength of binding of host range mutants to V. cholerae correlated with plaque morphology, indicating that the selected mutations in gp25 and gp23 restore molecular interactions with the receptor. We propose that ICP2 host range mutants evolve by a two-step process. First, gp25 mutations are selected for their broad host range, albeit accompanied by low-level phage adsorption. Subsequent selection occurs for gp23 mutations that further increase productive binding to specific OmpU alleles, allowing for near-wild-type efficiencies of adsorption and subsequent phage multiplication. IMPORTANCE Concern over multidrug-resistant bacterial pathogens, including Vibrio cholerae, has led to renewed interest in phage biology and the potential for phage therapy. ICP2 is a genetically unique virulent phage isolated from cholera patient stool samples. It is also one of three phages in a prophylactic cocktail that have been shown to be effective in animal models of infection and the only one of the three that requires a protein receptor (OmpU). This study identifies an ICP2 tail fiber and a receptor binding protein and examines how ICP2 responds to the selective pressures of phage-resistant OmpU mutants. We found that this particular coevolutionary arms race presents fitness costs to both ICP2 and V. cholerae.

Targeting of Mammalian Glycans Enhances Phage Predation in the Gastrointestinal Tract.

The human gastrointestinal mucosal surface consists of a eukaryotic epithelium, a prokaryotic microbiota, and a carbohydrate-rich interface that separates them. In the gastrointestinal tract, the interaction of bacteriophages (phages) and their prokaryotic hosts influences the health of the mammalian host, especially colonization with invasive pathobionts. Antibiotics may be used, but they also kill protective commensals. Here, we report a novel phage whose lytic cycle is enhanced in intestinal environments. The tail fiber gene, whose protein product binds human heparan sulfated proteoglycans and localizes the phage to the epithelial cell surface, positions it near its bacterial host, a type of locational targeting mechanism. This finding offers the prospect of developing mucosal targeting phage to selectively remove invasive pathobiont species from mucosal surfaces.IMPORTANCE Invasive pathobionts or microbes capable of causing disease can reside deep within the mucosal epithelium of our gastrointestinal tract. Targeted effective antibacterial therapies are needed to combat these disease-causing organisms, many of which may be multidrug resistant. Here, we isolated a lytic bacteriophage (phage) that can localize to the epithelial surface by binding heparan sulfated glycans, positioning it near its host, Escherichia coli This targeted therapy can be used to selectively remove invasive pathobionts from the gastrointestinal tract, preventing the development of disease.

Pantoea stewartii WceF is a glycan biofilm-modifying enzyme with a bacteriophage tailspike-like fold.

Pathogenic microorganisms often reside in glycan-based biofilms. Concentration and chain length distribution of these mostly anionic exopolysaccharides (EPS) determine the overall biophysical properties of a biofilm and result in a highly viscous environment. Bacterial communities regulate this biofilm state via intracellular small-molecule signaling to initiate EPS synthesis. Reorganization or degradation of this glycan matrix, however, requires the action of extracellular glycosidases. So far, these were mainly described for bacteriophages that must degrade biofilms for gaining access to host bacteria. The plant pathogen Pantoea stewartii (P. stewartii) encodes the protein WceF within its EPS synthesis cluster. WceF has homologs in various biofilm forming plant pathogens of the Erwinia family. In this work, we show that WceF is a glycosidase active on stewartan, the main P. stewartii EPS biofilm component. WceF has remarkable structural similarity with bacteriophage tailspike proteins (TSPs). Crystal structure analysis showed a native trimer of right-handed parallel ß-helices. Despite its similar fold, WceF lacks the high stability found in bacteriophage TSPs. WceF is a stewartan hydrolase and produces oligosaccharides, corresponding to single stewartan repeat units. However, compared with a stewartan-specific glycan hydrolase of bacteriophage origin, WceF showed lectin-like autoagglutination with stewartan, resulting in notably slower EPS cleavage velocities. This emphasizes that the bacterial enzyme WceF has a role in P. stewartii biofilm glycan matrix reorganization clearly different from that of a bacteriophage exopolysaccharide depolymerase.

Self-Competitive Inhibition of the Bacteriophage P22 Tailspike Endorhamnosidase by O-Antigen Oligosaccharides.

The P22 tailspike endorhamnosidase confers the high specificity of bacteriophage P22 for some serogroups of Salmonella differing only slightly in their O-antigen polysaccharide. We used several biophysical methods to study the binding and hydrolysis of O-antigen fragments of different lengths by P22 tailspike protein. O-Antigen saccharides of defined length labeled with fluorophors could be purified with higher resolution than previously possible. Small amounts of naturally occurring variations of O-antigen fragments missing the nonreducing terminal galactose could be used to determine the contribution of this part to the free energy of binding to be ∼7 kJ/mol. We were able to show via several independent lines of evidence that an unproductive binding mode is highly favored in binding over all other possible binding modes leading to hydrolysis. This is true even under circumstances under which the O-antigen fragment is long enough to be cleaved efficiently by the enzyme. The high-affinity unproductive binding mode results in a strong self-competitive inhibition in addition to product inhibition observed for this system. Self-competitive inhibition is observed for all substrates that have a free reducing end rhamnose. Naturally occurring O-antigen, while still attached to the bacterial outer membrane, does not have a free reducing end and therefore does not perform self-competitive inhibition.

Structures of the tailed bacteriophages that infect Gram-positive bacteria.

Productive virus infection depends upon delivery of viral genomic material into the host cell cytoplasm. The tails of bacteriophages recognize host cells and mediate host cell wall and membrane penetration. Recent cryo-electron microscopy studies have revealed near atomic-resolutions structures of the entire or almost entire bacteriophage particles of model systems including phi29, P22, P68, and T4. These structures allow comparisons between not only different states of the same phage but also between distantly related phages. In this review, we summarize the findings from recent structural studies of the bacteriophages that target Gram-positive bacteria, for a better understanding of the interactions between host cells and bacteriophages.

Bacteriophage Tail-Spike Proteins Enable Detection of Pseudaminic-Acid-Coated Pathogenic Bacteria and Guide the Development of Antiglycan Antibodies with Cross-Species Antibacterial Activity.

Pseudaminic acid (Pse), a unique carbohydrate in surface-associated glycans of pathogenic bacteria, has pivotal roles in virulence. Owing to its significant antigenicity and absence in mammals, Pse is considered an attractive target for vaccination or antibody-based therapies against bacterial infections. However, a specific and universal probe for Pse, which could also be used in immunotherapy, has not been reported. In a prior study, we used a tail spike protein from a bacteriophage (ΦAB6TSP) that digests Pse-containing exopolysaccharide (EPS) from Acinetobacter baumannii strain 54149 (Ab-54149) to form a glycoconjugate for preparing anti-Ab-54149 EPS serum. We report here that a catalytically inactive ΦAB6TSP (I-ΦAB6TSP) retains binding ability toward Pse. In addition, an I-ΦAB6TSP-DyLight-650 conjugate (Dy-I-ΦAB6TSP) was more sensitive in detecting Ab-54149 than an antibody purified from anti- Ab-54149 EPS serum. Dy-I-ΦAB6TSP also cross-reacted with other pathogenic bacteria containing Pse on their surface polysaccharides (e.g., Helicobacter pylori and Enterobacter cloacae), revealing it to be a promising probe for detecting Pse across bacterial species. We also developed a detection method that employs I-ΦAB6TSP immobilized on microtiter plate. These results suggested that the anti-Ab-54149 EPS serum would exhibit cross-reactivity to Pse on other organisms. When this was tested, this serum facilitated complement-mediated killing of H. pylori and E. cloacae, indicating its potential as a cross-species antibacterial agent. This work opens new avenues for diagnosis and treatment of multidrug resistant (MDR) bacterial infections.

The Central Spike Complex of Bacteriophage T4 Contacts PpiD in the Periplasm of Escherichia coli.

Infecting bacteriophage T4 uses a contractile tail structure to breach the envelope of the Escherichia coli host cell. During contraction, the tail tube headed with the "central spike complex" is thought to mechanically puncture the outer membrane. We show here that a purified tip fragment of the central spike complex interacts with periplasmic chaperone PpiD, which is anchored to the cytoplasmic membrane. PpiD may be involved in the penetration of the inner membrane by the T4 injection machinery, resulting in a DNA-conducting channel to translocate the phage DNA into the interior of the cell. Host cells with the ppiD gene deleted showed partial reduction in the plating efficiency of T4, suggesting a supporting role of PpiD to improve the efficiency of the infection process.

Structure and function of bacteriophage CBA120 ORF211 (TSP2), the determinant of phage specificity towards E. coli O157:H7.

The genome of Escherichia coli O157:H7 bacteriophage vB_EcoM_CBA120 encodes four distinct tailspike proteins (TSPs). The four TSPs, TSP1-4, attach to the phage baseplate forming a branched structure. We report the 1.9 Å resolution crystal structure of TSP2 (ORF211), the TSP that confers phage specificity towards E. coli O157:H7. The structure shows that the N-terminal 168 residues involved in TSPs complex assembly are disordered in the absence of partner proteins. The ensuing head domain contains only the first of two fold modules seen in other phage vB_EcoM_CBA120 TSPs. The catalytic site resides in a cleft at the interface between adjacent trimer subunits, where Asp506, Glu568, and Asp571 are located in close proximity. Replacement of Asp506 and Asp571 for alanine residues abolishes enzyme activity, thus identifying the acid/base catalytic machinery. However, activity remains intact when Asp506 and Asp571 are mutated into asparagine residues. Analysis of additional site-directed mutants in the background of the D506N:D571N mutant suggests engagement of an alternative catalytic apparatus comprising Glu568 and Tyr623. Finally, we demonstrate the catalytic role of two interacting glutamate residues of TSP1, located in a cleft between two trimer subunits, Glu456 and Glu483, underscoring the diversity of the catalytic apparatus employed by phage vB_EcoM_CBA120 TSPs.

Structural and functional characterization of the deep-sea thermophilic bacteriophage GVE2 tailspike protein.

The genome of the thermophilic bacteriophage GVE2 encodes a putative tailspike protein (GVE2 TSP). Here we report the crystal structure of the truncated GVE2 TSP at 2.0-Å resolution lacking 204 amino acid residues at its N-terminus (ΔnGVE2 TSP), possessing a "vase" outline similar to other TSP's structures. However, ΔnGVE2 TSP displays structural characteristics distinct from other TSPs. Despite lacking 204 amino acid residues, the head domain forms an asymmetric trimer compared to symmetric in other TSPs, suggesting that its long N-terminus may be unique to the long-tailed bacteriophages. Furthermore, the α-helix of the neck is 5-7 amino acids longer than that of other TSPs. The most striking feature is that its binding domain consists of a ß-helix with 10 turns, whereas other TSPs have 13 turns, even including the phage Sf6 TSP, which is the closest homologue of GVE2 TSP. The C-terminal structure is also quite different with those of other TSPs. Furthermore, we observed that ΔnGVE2 TSP can slow down growth of its host, demonstrating that this TSP is essential for the phage GVE2 to infect its host. Overall, the structural characteristics suggest that GVE2 TSP may be more primitive than other phage TSPs.

Structure, function and assembly of the long, flexible tail of siphophages.

Bacteriophages, viruses that infect bacteria, are the most abundant biological entities on Earth. Siphophages, accounting for ∼60% of known phages, bear a long, flexible tail that allows host recognition and safe delivery of the DNA from the capsid to the cytoplasm of the infected cell. Independently from their host (Gram positive or Gram negative) and the nature of their receptor at its surface (polysaccharide or protein), the core tail architecture of all caudophages and of bacterial phage-derived contractile injection systems share the same structural organisation and are thought to be homologous. Here, we review the recent advances in the structure, function and assembly of the core tail architecture of siphophages.

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.

Mutagenic Analysis of a DNA Translocating Tube's Interior Surface.

Bacteriophage ϕX174 uses a decamer of DNA piloting proteins to penetrate its host. These proteins oligomerize into a cell wall-spanning tube, wide enough for genome passage. While the inner surface of the tube is primarily lined with inward-facing amino acid side chains containing amide and guanidinium groups, there is a 28 Å-long section near the tube's C-terminus that does not exhibit this motif. The majority of the inward-facing residues in this region are conserved across the three ϕX174-like clades, suggesting that they play an important role during genome delivery. To test this hypothesis, and explore the general function of the tube's inner surface, non-glutamine residues within this region were mutated to glutamine, while existing glutamine residues were changed to serine. Four of the resulting mutants had temperature-dependent phenotypes. Virion assembly, host attachment, and virion eclipse, defined as the cell's ability to inactivate the virus, were not affected. Genome delivery, however, was inhibited. The results support a model in which a balance of forces governs genome delivery: potential energy provided by the densely packaged viral genome and/or an osmotic gradient move the genome into the cell, while the tube's inward facing glutamine residues exert a frictional force, or drag, that controls genome release.

Conserved and Diverse Traits of Adhesion Devices from Siphoviridae Recognizing Proteinaceous or Saccharidic Receptors.

Bacteriophages can play beneficial roles in phage therapy and destruction of food pathogens. Conversely, they play negative roles as they infect bacteria involved in fermentation, resulting in serious industrial losses. Siphoviridae phages possess a long non-contractile tail and use a mechanism of infection whose first step is host recognition and binding. They have evolved adhesion devices at their tails' distal end, tuned to recognize specific proteinaceous or saccharidic receptors on the host's surface that span a large spectrum of shapes. In this review, we aimed to identify common patterns beyond this apparent diversity. To this end, we analyzed siphophage tail tips or baseplates, evaluating their known structures, where available, and uncovering patterns with bioinformatics tools when they were not. It was thereby identified that a triad formed by three proteins in complex, i.e., the tape measure protein (TMP), the distal tail protein (Dit), and the tail-associated lysozyme (Tal), is conserved in all phages. This common scaffold may harbor various functional extensions internally while it also serves as a platform for plug-in ancillary or receptor-binding proteins (RBPs). Finally, a group of siphophage baseplates involved in saccharidic receptor recognition exhibits an activation mechanism reminiscent of that observed in Myoviridae.