NMR detection and conformational dependence of two, three, and four-bond isotope shifts due to deuteration of backbone amides.

NMR isotope shifts occur due to small differences in nuclear shielding when nearby atoms are different isotopes. For molecules dissolved in 1:1 H2O:D2O, the resulting mixture of N-H and N-D isotopes leads to a small splitting of resonances from adjacent nuclei. We used multidimensional NMR to measure isotope shifts for the proteins CUS-3iD and CspA. We observed four-bond 4∆N(ND) isotope shifts in high-resolution 2D 15N-TROSY experiments of the perdeuterated proteins that correlate with the torsional angle psi. Three-bond 3∆C'(ND) isotope shifts detected in H(N)CO spectra correlate with the intraresidue H-O distance, and to a lesser extent with the dihedral angle phi. The conformational dependence of the isotope shifts agree with those previously reported in the literature. Both the 4∆N(ND) and 3∆C'(ND) isotope shifts are sensitive to distances between the atoms giving rise to the isotope shifts and the atoms experiencing the splitting, however, these distances are strongly correlated with backbone dihedral angles making it difficult to resolve distance from stereochemical contributions to the isotope shift. H(NCA)CO spectra were used to measure two-bond 2∆C'(ND) isotope shifts and [D]/[H] fractionation factors. Neither parameter showed significant differences for hydrogen-bonded sites, or changes over a 25° temperature range, suggesting they are not sensitive to hydrogen bonding. Finally, the quartet that arises from the combination of 2∆C'(ND) and 3∆C'(ND) isotope shifts in H(CA)CO spectra was used to measure synchronized hydrogen exchange for the sequence neighbors A315-S316 in the protein CUS-3iD. In many of our experiments we observed minor resonances due to the 10% D2O used for the sample deuterium lock, indicating isotope shifts can be a source of spectral heterogeneity in standard NMR experiments. We suggest that applications of isotope shifts such as conformational analysis and correlated hydrogen exchange could benefit from the larger magnetic fields becoming available.

Architect of Virus Assembly: the Portal Protein Nucleates Procapsid Assembly in Bacteriophage P22.

Tailed double-stranded DNA (dsDNA) bacteriophages, herpesviruses, and adenoviruses package their genetic material into a precursor capsid through a dodecameric ring complex called the portal protein, which is located at a unique 5-fold vertex. In several phages and viruses, including T4, Φ29, and herpes simplex virus 1 (HSV-1), the portal forms a nucleation complex with scaffolding proteins (SPs) to initiate procapsid (PC) assembly, thereby ensuring incorporation of only one portal ring per capsid. However, for bacteriophage P22, the role of its portal protein in initiation of procapsid assembly is unclear. We have developed an in vitro P22 assembly assay where portal protein is coassembled into procapsid-like particles (PLPs). Scaffolding protein also catalyzes oligomerization of monomeric portal protein into dodecameric rings, possibly forming a scaffolding protein-portal protein nucleation complex that results in one portal ring per P22 procapsid. Here, we present evidence substantiating that the P22 portal protein, similarly to those of other dsDNA viruses, can act as an assembly nucleator. The presence of the P22 portal protein is shown to increase the rate of particle assembly and contribute to proper morphology of the assembled particles. Our results highlight a key function of portal protein as an assembly initiator, a feature that is likely conserved among these classes of dsDNA viruses.IMPORTANCE The existence of a single portal ring is essential to the formation of infectious virions in the tailed double-stranded DNA (dsDNA) phages, herpesviruses, and adenoviruses and, as such, is a viable antiviral therapeutic target. How only one portal is selectively incorporated at a unique vertex is unclear. In many dsDNA viruses and phages, the portal protein acts as an assembly nucleator. However, early work on phage P22 assembly in vivo indicated that the portal protein did not function as a nucleator for procapsid (PC) assembly, leading to the suggestion that P22 uses a unique mechanism for portal incorporation. Here, we show that portal protein nucleates assembly of P22 procapsid-like particles (PLPs). Addition of portal rings to an assembly reaction increases the rate of formation and yield of particles and corrects improper particle morphology. Our data suggest that procapsid assembly may universally initiate with a nucleation complex composed minimally of portal and scaffolding proteins (SPs).

A Hydrophobic Network: Intersubunit and Intercapsomer Interactions Stabilizing the Bacteriophage P22 Capsid.

Double-stranded DNA (dsDNA) tailed phages and herpesviruses assemble their capsids using coat proteins that have the ubiquitous HK97 fold. Though this fold is common, we do not have a thorough understanding of the different ways viruses adapt it to maintain stability in various environments. The HK97-fold E-loop, which connects adjacent subunits at the outer periphery of capsomers, has been implicated in capsid stability. Here, we show that in bacteriophage P22, residue W61 at the tip of the E-loop plays a role in stabilizing procapsids and in maturation. We hypothesize that a hydrophobic pocket is formed by residues I366 and W410 in the P domain of a neighboring subunit within a capsomer, into which W61 fits like a peg. In addition, W61 likely bridges to residues A91 and L401 in P-domain loops of an adjacent capsomer, thereby linking the entire capsid together with a network of hydrophobic interactions. There is conservation of this hydrophobic network in the distantly related P22-like phages, indicating that this structural feature is likely important for stabilizing this family of phages. Thus, our data shed light on one of the varied elegant mechanisms used in nature to consistently build stable viral genome containers through subtle adaptation of the HK97 fold.IMPORTANCE Similarities in assembly reactions and coat protein structures of the dsDNA tailed phages and herpesviruses make phages ideal models to understand capsid assembly and identify potential targets for antiviral drug discovery. The coat protein E-loops of these viruses are involved in both intra- and intercapsomer interactions. In phage P22, hydrophobic interactions peg the coat protein subunits together within a capsomer, where the E-loop hydrophobic residue W61 of one subunit packs into a pocket of hydrophobic residues I366 and W410 of the adjacent subunit. W61 also makes hydrophobic interactions with A91 and L401 of a subunit in an adjacent capsomer. We show these intra- and intercapsomer hydrophobic interactions form a network crucial to capsid stability and proper assembly.

Conservation and Divergence of the I-Domain Inserted into the Ubiquitous HK97 Coat Protein Fold in P22-Like Bacteriophages.

Despite very low sequence homology, the major capsid proteins of double-stranded DNA (dsDNA) bacteriophages, some archaeal viruses, and the herpesviruses share a structural motif, the HK97 fold. Bacteriophage P22, a paradigm for this class of viruses, belongs to a phage gene cluster that contains three homology groups: P22-like, CUS-3-like, and Sf6-like. The coat protein of each phage has an inserted domain (I-domain) that is more conserved than the rest of the coat protein. In P22, loops in the I-domain are critical for stabilizing intra- and intersubunit contacts that guide proper capsid assembly. The nuclear magnetic resonance (NMR) structures of the P22, CUS-3, and Sf6 I-domains reveal that they are all six-stranded, anti-parallel ß-barrels. Nevertheless, significant structural differences occur in loops connecting the ß-strands, in surface electrostatics used to dock the I-domains with their respective coat protein core partners, and in sequence motifs displayed on the capsid surfaces. Our data highlight the structural diversity of I-domains that could lead to variations in capsid assembly mechanisms and capsid surfaces adapted for specific phage functions.IMPORTANCE Comparative studies of protein structures often provide insights into their evolution. The HK97 fold is a structural motif used to form the coat protein shells that encapsidate the genomes of many dsDNA phages and viruses. The structure and function of coat proteins based on the HK97 fold are often embellished by the incorporation of I-domains. In the present work we compare I-domains from three phages representative of highly divergent P22-like homology groups. While the three I-domains share a six-stranded ß-barrel skeleton, there are differences (i) in structure elements at the periphery of the conserved fold, (ii) in the locations of disordered loops important in capsid assembly and conformational transitions, (iii) in surfaces charges, and (iv) in sequence motifs that are potential ligand-binding sites. These structural modifications on the rudimentary I-domain fold suggest that considerable structural adaptability was needed to fulfill the versatile range of functional requirements for distinct phages.

NMR Mapping of Disordered Segments from a Viral Scaffolding Protein Enclosed in a 23 MDa Procapsid.

Scaffolding proteins (SPs) are required for the capsid shell assembly of many tailed double-stranded DNA bacteriophages, some archaeal viruses, herpesviruses, and adenoviruses. Despite their importance, only one high-resolution structure is available for SPs within procapsids. Here, we use the inherent size limit of NMR to identify mobile segments of the 303-residue phage P22 SP free in solution and when incorporated into a ∼23 MDa procapsid complex. Free SP gives NMR signals from its acidic N-terminus (residues 1-40) and basic C-terminus (residues 264-303), whereas NMR signals from the middle segment (residues 41-263) are missing because of intermediate conformational exchange on the NMR chemical shift timescale. When SP is incorporated into P22 procapsids, NMR signals from the C-terminal helix-turn-helix domain disappear because of binding to the procapsid interior. Signals from the N-terminal domain persist, indicating that this segment retains flexibility when bound to procapsids. The unstructured character of the N-terminus, coupled with its high content of negative charges, is likely important for dissociation and release of SP during the double-stranded DNA genome packaging step accompanying phage maturation.

Contextual Role of a Salt Bridge in the Phage P22 Coat Protein I-Domain.

The I-domain is a genetic insertion in the phage P22 coat protein that chaperones its folding and stability. Of 11 acidic residues in the I-domain, seven participate in stabilizing electrostatic interactions with basic residues across elements of secondary structure, fastening the ß-barrel fold. A hydrogen-bonded salt bridge between Asp-302 and His-305 is particularly interesting as Asp-302 is the site of a temperature-sensitive-folding mutation. The pKa of His-305 is raised to 9.0, indicating the salt bridge stabilizes the I-domain by ∼4 kcal/mol. Consistently, urea denaturation experiments indicate the stability of the WT I-domain decreases by 4 kcal/mol between neutral and basic pH. The mutants D302A and H305A remove the pH dependence of stability. The D302A substitution destabilizes the I-domain by 4 kcal/mol, whereas H305A had smaller effects, on the order of 1-2 kcal/mol. The destabilizing effects of D302A are perpetuated in the full-length coat protein as shown by a higher sensitivity to protease digestion, decreased procapsid assembly rates, and impaired phage production in vivo By contrast, the mutants have only minor effects on capsid expansion or stability in vitro The effects of the Asp-302-His-305 salt bridge are thus complex and context-dependent. Substitutions that abolish the salt bridge destabilize coat protein monomers and impair capsid self-assembly, but once capsids are formed the effects of the substitutions are overcome by new quaternary interactions between subunits.

A Molecular Staple: D-Loops in the I Domain of Bacteriophage P22 Coat Protein Make Important Intercapsomer Contacts Required for Procapsid Assembly.

UNLABELLED: Bacteriophage P22, a double-stranded DNA (dsDNA) virus, has a nonconserved 124-amino-acid accessory domain inserted into its coat protein, which has the canonical HK97 protein fold. This I domain is involved in virus capsid size determination and stability, as well as protein folding. The nuclear magnetic resonance (NMR) solution structure of the I domain revealed the presence of a D-loop, which was hypothesized to make important intersubunit contacts between coat proteins in adjacent capsomers. Here we show that amino acid substitutions of residues near the tip of the D-loop result in aberrant assembly products, including tubes and broken particles, highlighting the significance of the D-loops in proper procapsid assembly. Using disulfide cross-linking, we showed that the tips of the D-loops are positioned directly across from each other both in the procapsid and the mature virion, suggesting their importance in both states. Our results indicate that D-loop interactions act as "molecular staples" at the icosahedral 2-fold symmetry axis and significantly contribute to stabilizing the P22 capsid for DNA packaging. IMPORTANCE: Many dsDNA viruses have morphogenic pathways utilizing an intermediate capsid, known as a procapsid. These procapsids are assembled from a coat protein having the HK97 fold in a reaction driven by scaffolding proteins or delta domains. Maturation of the capsid occurs during DNA packaging. Bacteriophage HK97 uniquely stabilizes its capsid during maturation by intercapsomer cross-linking, but most virus capsids are stabilized by alternate means. Here we show that the I domain that is inserted into the coat protein of bacteriophage P22 is important in the process of proper procapsid assembly. Specifically, the I domain allows for stabilizing interactions across the capsid 2-fold axis of symmetry via a D-loop. When amino acid residues at the tip of the D-loop are mutated, aberrant assembly products, including tubes, are formed instead of procapsids, consequently phage production is affected, indicating the importance of stabilizing interactions during the assembly and maturation reactions.

Measurement of the accurate mass of a 50 MDa infectious virus.

RATIONALE: Bacteriophage P22 is believed to contain a total of 521 copies of 9 different proteins and a 41,724 base pair genome. Despite its enormous size and complexity, phage P22 can be electrosprayed, and it remains intact in ultra-high vacuum where its molar mass distribution has been measured. METHODS: Phage P22 virions were generated by complementation in Salmonella enterica and purified. They were transferred into 100 mM ammonium acetate and then electrosprayed. The masses of individual virions were determined using charge detection mass spectrometry. RESULTS: The stoichiometry of the protein components of phage P22 is sufficiently well known that the theoretical molar mass can be determined to within a narrow range. The measured average molar mass of phage P22, 52,180 ± 59 kDa, is consistent with the theoretical molar mass and supports the proposed stoichiometry of the components. The intrinsic width of the phage P22 mass distribution can be accounted for by the distribution of DNA packaged by the headful mechanism. CONCLUSIONS: At over 50 MDa, phage P22 is the largest object with a well-defined molar mass to be analyzed by mass spectrometry. The narrow measured mass distribution indicates that the virions survive the transition into the gas phase intact. Copyright © 2016 John Wiley & Sons, Ltd.

Mechanism of Protein Denaturation: Partial Unfolding of the P22 Coat Protein I-Domain by Urea Binding.

The I-domain is an insertion domain of the bacteriophage P22 coat protein that drives rapid folding and accounts for over half of the stability of the full-length protein. We sought to determine the role of hydrogen bonds (H-bonds) in the unfolding of the I-domain by examining (3)JNC' couplings transmitted through H-bonds, the temperature and urea-concentration dependence of (1)HN and (15)N chemical shifts, and native-state hydrogen exchange at urea concentrations where the domain is predominantly folded. The native-state hydrogen-exchange data suggest that the six-stranded ß-barrel core of the I-domain is more stable against unfolding than a smaller subdomain comprised of a short α-helix and three-stranded ß-sheet. H-bonds, separately determined from solvent protection and (3)JNC' H-bond couplings, are identified with an accuracy of 90% by (1)HN temperature coefficients. The accuracy is improved to 95% when (15)N temperature coefficients are also included. In contrast, the urea dependence of (1)HN and (15)N chemical shifts is unrelated to H-bonding. The protein segments with the largest chemical-shift changes in the presence of urea show curved or sigmoidal titration curves suggestive of direct urea binding. Nuclear Overhauser effects to urea for these segments are also consistent with specific urea-binding sites in the I-domain. Taken together, the results support a mechanism of urea unfolding in which denaturant binds to distinct sites in the I-domain. Disordered segments bind urea more readily than regions in stable secondary structure. The locations of the putative urea-binding sites correlate with the lower stability of the structure against solvent exchange, suggesting that partial unfolding of the structure is related to urea accessibility.

ADP-dependent conformational changes distinguish Mycobacterium tuberculosis SecA2 from SecA1.

In bacteria, most secreted proteins are exported through the SecYEG translocon by the SecA ATPase motor via the general secretion or "Sec" pathway. The identification of an additional SecA protein, particularly in Gram-positive pathogens, has raised important questions about the role of SecA2 in both protein export and establishment of virulence. We previously showed in Mycobacterium tuberculosis, the causative agent of tuberculosis, the accessory SecA2 protein possesses ATPase activity that is required for bacterial survival in host macrophages, highlighting its importance in virulence. Here, we show that SecA2 binds ADP with much higher affinity than SecA1 and releases the nucleotide more slowly. Nucleotide binding also regulates movement of the precursor-binding domain in SecA2, unlike in SecA1 or conventional SecA proteins. This conformational change involving closure of the clamp in SecA2 may provide a mechanism for the cell to direct protein export through the conventional SecA1 pathway under normal growth conditions while preventing ordinary precursor proteins from interacting with the specialized SecA2 ATPase.

Highly specific salt bridges govern bacteriophage P22 icosahedral capsid assembly: identification of the site in coat protein responsible for interaction with scaffolding protein.

UNLABELLED: Icosahedral virus assembly requires a series of concerted and highly specific protein-protein interactions to produce a proper capsid. In bacteriophage P22, only coat protein (gp5) and scaffolding protein (gp8) are needed to assemble a procapsid-like particle, both in vivo and in vitro. In scaffolding protein's coat binding domain, residue R293 is required for procapsid assembly, while residue K296 is important but not essential. Here, we investigate the interaction of scaffolding protein with acidic residues in the N-arm of coat protein, since this interaction has been shown to be electrostatic. Through site-directed mutagenesis of genes 5 and 8, we show that changing coat protein N-arm residue 14 from aspartic acid to alanine causes a lethal phenotype. Coat protein residue D14 is shown by cross-linking to interact with scaffolding protein residue R293 and, thus, is intimately involved in proper procapsid assembly. To a lesser extent, coat protein N-arm residue E18 is also implicated in the interaction with scaffolding protein and is involved in capsid size determination, since a cysteine mutation at this site generated petite capsids. The final acidic residue in the N-arm that was tested, E15, is shown to only weakly interact with scaffolding protein's coat binding domain. This work supports growing evidence that surface charge density may be the driving force of virus capsid protein interactions. IMPORTANCE: Bacteriophage P22 infects Salmonella enterica serovar Typhimurium and is a model for icosahedral viral capsid assembly. In this system, coat protein interacts with an internal scaffolding protein, triggering the assembly of an intermediate called a procapsid. Previously, we determined that there is a single amino acid in scaffolding protein required for P22 procapsid assembly, although others modulate affinity. Here, we identify partners in coat protein. We show experimentally that relatively weak interactions between coat and scaffolding proteins are capable of driving correctly shaped and sized procapsids and that the lack of these proper protein-protein interfaces leads to aberrant structures. The present work represents an important contribution supporting the hypothesis that virus capsid assembly is governed by seemingly simple interactions. The highly specific nature of the subunit interfaces suggests that these could be good targets for antivirals.

A method to investigate protein association with intact sealed mycobacterial membrane vesicles.

In mycobacteria, probing the association of cytoplasmic proteins with the membrane itself, as well as with integral or peripheral membrane proteins, is limited by the difficulty in extracting intact sealed membrane vesicles due to the complex cell wall structure. Here we tested the association of Mycobacterium tuberculosis SecA1 and SecA2 proteins with intact membrane vesicles by a flotation assay using iodixanol density gradients. These protocols have wide applications for studying the association of other mycobacterial cytoplasmic proteins with the membrane and membrane-associated proteins.

An intramolecular chaperone inserted in bacteriophage P22 coat protein mediates its chaperonin-independent folding.

The bacteriophage P22 coat protein has the common HK97-like fold but with a genetically inserted domain (I-domain). The role of the I-domain, positioned at the outermost surface of the capsid, is unknown. We hypothesize that the I-domain may act as an intramolecular chaperone because the coat protein folds independently, and many folding mutants are localized to the I-domain. The function of the I-domain was investigated by generating the coat protein core without its I-domain and the isolated I-domain. The core coat protein shows a pronounced folding defect. The isolated I-domain folds autonomously and has a high thermodynamic stability and fast folding kinetics in the presence of a peptidyl prolyl isomerase. Thus, the I-domain provides thermodynamic stability to the full-length coat protein so that it can fold reasonably efficiently while still allowing the HK97-like core to retain the flexibility required for conformational switching during procapsid assembly and maturation.

Templated trimerization of the phage L decoration protein on capsids.

The 134-residue phage L decoration protein (Dec) forms a capsid-stabilizing homotrimer that has an asymmetric tripod-like structure when bound to phage L capsids. The N-termini of the trimer subunits consist of spatially separated globular OB-fold domains that interact with the virions of phage L or the related phage P22. The C-termini of the trimer form a three-stranded intertwined spike structure that accounts for nearly all the interactions that stabilize the trimer. A Dec mutant with the spike residues 99-134 deleted (Dec 1-98 ) was used to demonstrate that the stable globular OB-fold domain folds independently of the C-terminal residues. However, Dec 1-98 was unable to bind phage P22 virions, indicating the C-terminal spike is essential for stable capsid interaction. The full-length Dec trimer is disassembled into monomers by acidification to pH <2. These monomers retain the folded globular OB-fold domain structure, but the spike is unfolded. Increasing the pH of the Dec monomer solution to pH 6 allowed for slow trimer formation in vitro over the course of days. The infectious cycle of phage L is only around an hour, however, implying Dec trimer assembly in vivo is templated by the phage capsid. The Thermodynamic Hypothesis holds that protein folding is determined by the amino acid sequence. Dec serves as an unusual example of an oligomeric folding step that is kinetically accelerated by a viral capsid template. The capsid templating mechanism could satisfy the flexibility needed for Dec to adapt to the unusual quasi-symmetric binding site on the mature phage L capsid.

Bacteriophage P22 SieA-mediated superinfection exclusion.

Many temperate phages encode prophage-expressed functions that interfere with superinfection of the host bacterium by external phages. Salmonella phage P22 has four such systems that are expressed from the prophage in a lysogen that are encoded by the c2 (repressor), gtrABC, sieA, and sieB genes. Here we report that the P22-encoded SieA protein is necessary and sufficient for exclusion by the SieA system and that it is an inner membrane protein that blocks DNA injection by P22 and its relatives, but has no effect on infection by other tailed phage types. The P22 virion injects its DNA through the host cell membranes and periplasm via a conduit assembled from three "ejection proteins" after their release from the virion. Phage P22 mutants that overcome the SieA block were isolated, and they have amino acid changes in the C-terminal regions of the gene 16 and 20 encoded ejection proteins. Three different single-amino acid changes in these proteins are required to obtain nearly full resistance to SieA. Hybrid P22 phages that have phage HK620 ejection protein genes are also partially resistant to SieA. There are three sequence types of extant phage-encoded SieA proteins that are less than 30% identical to one another, yet comparison of two of these types found no differences in phage target specificity. Our data strongly suggest a model in which the inner membrane protein SieA interferes with the assembly or function of the periplasmic gp20 and membrane-bound gp16 DNA delivery conduit.IMPORTANCEThe ongoing evolutionary battle between bacteria and the viruses that infect them is a critical feature of bacterial ecology on Earth. Viruses can kill bacteria by infecting them. However, when their chromosomes are integrated into a bacterial genome as a prophage, viruses can also protect the host bacterium by expressing genes whose products defend against infection by other viruses. This defense property is called "superinfection exclusion." A significant fraction of bacteria harbor prophages that encode such protective systems, and there are many different molecular strategies by which superinfection exclusion is mediated. This report is the first to describe the mechanism by which bacteriophage P22 SieA superinfection exclusion protein protects its host bacterium from infection by other P22-like phages. The P22 prophage-encoded inner membrane SieA protein prevents infection by blocking transport of superinfecting phage DNA across the inner membrane during injection.

Unraveling the role of the C-terminal helix turn helix of the coat-binding domain of bacteriophage P22 scaffolding protein.

Many viruses encode scaffolding and coat proteins that co-assemble to form procapsids, which are transient precursor structures leading to progeny virions. In bacteriophage P22, the association of scaffolding and coat proteins is mediated mainly by ionic interactions. The coat protein-binding domain of scaffolding protein is a helix turn helix structure near the C terminus with a high number of charged surface residues. Residues Arg-293 and Lys-296 are particularly important for coat protein binding. The two helices contact each other through hydrophobic side chains. In this study, substitution of the residues of the interface between the helices, and the residues in the ß-turn, by aspartic acid was used examine the importance of the conformation of the domain in coat binding. These replacements strongly affected the ability of the scaffolding protein to interact with coat protein. The severity of the defect in the association of scaffolding protein to coat protein was dependent on location, with substitutions at residues in the turn and helix 2 causing the most significant effects. Substituting aspartic acid for hydrophobic interface residues dramatically perturbs the stability of the structure, but similar substitutions in the turn had much less effect on the integrity of this domain, as determined by circular dichroism. We propose that the binding of scaffolding protein to coat protein is dependent on angle of the ß-turn and the orientation of the charged surface on helix 2. Surprisingly, formation of the highly complex procapsid structure depends on a relatively simple interaction.

Bacteriophage P22 SieA mediated superinfection exclusion.

Many temperate phages encode prophage-expressed functions that interfere with superinfection of the host bacterium by external phages. Salmonella phage P22 has four such systems that are expressed from the prophage in a lysogen that are encoded by the c2 (repressor), gtrABC, sieA, and sieB genes. Here we report that the P22-encoded SieA protein is the only phage protein required for exclusion by the SieA system, and that it is an inner membrane protein that blocks DNA injection by P22 and its relatives, but has no effect on infection by other tailed phage types. The P22 virion injects its DNA through the host cell membranes and periplasm via a conduit assembled from three "ejection proteins" after their release from the virion. Phage P22 mutants were isolated that overcome the SieA block, and they have amino acid changes in the C-terminal regions of the gene 16 and 20 encoded ejection proteins. Three different single amino acid changes in these proteins are required to obtain nearly full resistance to SieA. Hybrid P22 phages that have phage HK620 ejection protein genes are also partially resistant to SieA. There are three sequence types of extant phage-encoded SieA proteins that are less than 30% identical to one another, yet comparison of two of these types found no differences in target specificity. Our data are consistent with a model in which the inner membrane protein SieA interferes with the assembly or function of the periplasmic gp20 and membrane-bound gp16 DNA delivery conduit.

Molecular Architecture of Salmonella Typhimurium Virus P22 Genome Ejection Machinery.

Bacteriophage P22 is a prototypical member of the Podoviridae superfamily. Since its discovery in 1952, P22 has become a paradigm for phage transduction and a model for icosahedral viral capsid assembly. Here, we describe the complete architecture of the P22 tail apparatus (gp1, gp4, gp10, gp9, and gp26) and the potential location and organization of P22 ejection proteins (gp7, gp20, and gp16), determined using cryo-EM localized reconstruction, genetic knockouts, and biochemical analysis. We found that the tail apparatus exists in two equivalent conformations, rotated by ∼6° relative to the capsid. Portal protomers make unique contacts with coat subunits in both conformations, explaining the 12:5 symmetry mismatch. The tail assembles around the hexameric tail hub (gp10), which folds into an interrupted ß-propeller characterized by an apical insertion domain. The tail hub connects proximally to the dodecameric portal protein and head-to-tail adapter (gp4), distally to the trimeric tail needle (gp26), and laterally to six trimeric tailspikes (gp9) that attach asymmetrically to gp10 insertion domain. Cryo-EM analysis of P22 mutants lacking the ejection proteins gp7 or gp20 and biochemical analysis of purified recombinant proteins suggest that gp7 and gp20 form a molecular complex associated with the tail apparatus via the portal protein barrel. We identified a putative signal transduction pathway from the tailspike to the tail needle, mediated by three flexible loops in the tail hub, that explains how lipopolysaccharide (LPS) is sufficient to trigger the ejection of the P22 DNA in vitro.