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 the Branched Receptor-Binding Complex of Bacteriophage CBA120.

Bacteriophages recognize their host cells with the help of tail fiber and tailspike proteins that bind, cleave, or modify certain structures on the cell surface. The spectrum of ligands to which the tail fibers and tailspikes can bind is the primary determinant of the host range. Bacteriophages with multiple tailspike/tail fibers are thought to have a wider host range than their less endowed relatives but the function of these proteins remains poorly understood. Here, we describe the structure, function, and substrate specificity of three tailspike proteins of bacteriophage CBA120-TSP2, TSP3 and TSP4 (orf211 through orf213, respectively). We show that tailspikes TSP2, TSP3 and TSP4 are hydrolases that digest the O157, O77, and O78 Escherichia coli O-antigens, respectively. We demonstrate that recognition of the E. coli O157:H7 host by CBA120 involves binding to and digesting the O157 O-antigen by TSP2. We report the crystal structure of TSP2 in complex with a repeating unit of the O157 O-antigen. We demonstrate that according to the specificity of its tailspikes TSP2, TSP3, and TSP4, CBA120 can infect E. coli O157, O77, and O78, respectively. We also show that CBA120 infects Salmonella enterica serovar Minnesota, and this host range expansion is likely due to the function of TSP1. Finally, we describe the assembly pathway and the architecture of the TSP1-TSP2-TSP3-TSP4 branched complex in CBA120 and its related ViI-like phages.

Structure of an Acinetobacter Broad-Range Prophage Endolysin Reveals a C-Terminal α-Helix with the Proposed Role in Activity against Live Bacterial Cells.

Proteins that include enzymatic domain degrading the bacterial cell wall and a domain providing transport through the bacterial outer membrane are considered as prospective compounds to combat pathogenic Gram-negative bacteria. This paper presents an isolation and study of an enzyme of this class naturally encoded in the prophage region of Acinetobacter baumannii AB 5075 genome. Recombinant protein expressed in E. coli exhibits an antimicrobial activity with respect to live cultures of Gram-negative bacteria reducing the population of viable bacteria by 1.5⁻2 log colony forming units (CFU)/mL. However the protein becomes rapidly inactivated and enables the bacteria to restore the population. AcLys structure determined by X-ray crystallography reveals a predominantly α—helical fold similar to bacteriophage P22 lysozyme. The С-terminal part of AcLys polypeptide chains forms an α—helix enriched by Lys and Arg residues exposed outside of the protein globule. Presumably this type of structure of the C-terminal α—helix has evolved evolutionally enabling the endolysin to pass the inner membrane during the host lysis or, potentially, to penetrate the outer membrane of the Gram-negative bacteria.

Structure of the T4 baseplate and its function in triggering sheath contraction.

Several systems, including contractile tail bacteriophages, the type VI secretion system and R-type pyocins, use a multiprotein tubular apparatus to attach to and penetrate host cell membranes. This macromolecular machine resembles a stretched, coiled spring (or sheath) wound around a rigid tube with a spike-shaped protein at its tip. A baseplate structure, which is arguably the most complex part of this assembly, relays the contraction signal to the sheath. Here we present the atomic structure of the approximately 6-megadalton bacteriophage T4 baseplate in its pre- and post-host attachment states and explain the events that lead to sheath contraction in atomic detail. We establish the identity and function of a minimal set of components that is conserved in all contractile injection systems and show that the triggering mechanism is universally conserved.

Structure and Biophysical Properties of a Triple-Stranded Beta-Helix Comprising the Central Spike of Bacteriophage T4.

Gene product 5 (gp5) of bacteriophage T4 is a spike-shaped protein that functions to disrupt the membrane of the target cell during phage infection. Its C-terminal domain is a long and slender ß-helix that is formed by three polypeptide chains wrapped around a common symmetry axis akin to three interdigitated corkscrews. The folding and biophysical properties of such triple-stranded ß-helices, which are topologically related to amyloid fibers, represent an unsolved biophysical problem. Here, we report structural and biophysical characterization of T4 gp5 ß-helix and its truncated mutants of different lengths. A soluble fragment that forms a dimer of trimers and that could comprise a minimal self-folding unit has been identified. Surprisingly, the hydrophobic core of the ß-helix is small. It is located near the C-terminal end of the ß-helix and contains a centrally positioned and hydrated magnesium ion. A large part of the ß-helix interior comprises a large elongated cavity that binds palmitic, stearic, and oleic acids in an extended conformation suggesting that these molecules might participate in the folding of the complete ß-helix.

Crystal structure and location of gp131 in the bacteriophage phiKZ virion.

Pseudomonas phage ϕKZ and its two close relatives ϕPA3 and 201ϕ2-1 are very large bacteriophages that form a separate branch in phage classification because their genomes are very different from the rest of GenBank sequence data. The contractile tail of ϕKZ is built from at least 32 different proteins, but a definitive structural function is assigned to only one of them-the tail sheath protein. Here, we report the crystal structure of the C-terminal domain of another phiKZ tail protein, gene product 131 (gp131C). We show that gp131 is located at the periphery of the baseplate and possibly associates with fibers that emanate from the baseplate. Gp131C is a seven-bladed ß-propeller that has a shape of a skewed toroid. A small but highly conserved and negatively charged patch on the surface of gp131C might be important for substrate binding or for interaction with a different tail protein.

Phage pierces the host cell membrane with the iron-loaded spike.

Bacteriophages with contractile tails and the bacterial type VI secretion system have been proposed to use a special protein to create an opening in the host cell membrane during infection. These proteins have a modular architecture but invariably contain an oligonucleotide/oligosaccharide-binding (OB-fold) domain and a long ß-helical C-terminal domain, which initiates the contact with the host cell membrane. Using X-ray crystallography and electron microscopy, we report the atomic structure of the membrane-piercing proteins from bacteriophages P2 and ϕ92 and identify the residues that constitute the membrane-attacking apex. Both proteins form compact spikes with a ∼10Å diameter tip that is stabilized by a centrally positioned iron ion bound by six histidine residues. The accumulated data strongly suggest that, in the process of membrane penetration, the spikes are translocated through the lipid bilayer without undergoing major unfolding.

Structural and functional similarities between the capsid proteins of bacteriophages T4 and HK97 point to a common ancestry.

Gene product (gp) 24 of bacteriophage T4 forms the pentameric vertices of the capsid. Using x-ray crystallography, we found the principal domain of gp24 to have a polypeptide fold similar to that of the HK97 phage capsid protein plus an additional insertion domain. Fitting gp24 monomers into a cryo-EM density map of the mature T4 capsid suggests that the insertion domain interacts with a neighboring subunit, effecting a stabilization analogous to the covalent crosslinking in the HK97 capsid. Sequence alignment and genetic data show that the folds of gp24 and the hexamer-forming capsid protein, gp23*, are similar. Accordingly, models of gp24* pentamers, gp23* hexamers, and the whole capsid were built, based on a cryo-EM image reconstruction of the capsid. Mutations in gene 23 that affect capsid shape map to the capsomer's periphery, whereas mutations that allow gp23 to substitute for gp24 at the vertices modify the interactions between monomers within capsomers. Structural data show that capsid proteins of most tailed phages, and some eukaryotic viruses, may have evolved from a common ancestor.