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.

No abstract

Bacteriophages recognize and bind to their hosts with the help of receptor-binding proteins (RBPs) that emanate from the phage particle in the form of fibers or tailspikes. RBPs show a great variability in their shapes, sizes, and location on the particle. Some RBPs are known to depolymerize surface polysaccharides of the host while others show no enzymatic activity. Here we report that both RBPs of podovirus G7C - tailspikes gp63.1 and gp66 - are essential for infection of its natural host bacterium E. coli 4s that populates the equine intestinal tract. We characterize the structure and function of gp63.1 and show that unlike any previously described RPB, gp63.1 deacetylates surface polysaccharides of E. coli 4s leaving the backbone of the polysaccharide intact. We demonstrate that gp63.1 and gp66 form a stable complex, in which the N-terminal part of gp66 serves as an attachment site for gp63.1 and anchors the gp63.1-gp66 complex to the G7C tail. The esterase domain of gp63.1 as well as domains mediating the gp63.1-gp66 interaction is widespread among all three families of tailed bacteriophages.

The O antigen (O polysaccharide), composed of many oligosaccharide repeats (O units), is a part of the lipopolysaccharide (LPS) of Gram-negative bacteria and the most structurally variable cell surface constituent. The O-antigen diversity is due to variations of O-antigen biosynthesis genes and is believed to offer various bacterial clones selective advantages in their specific ecological niches (1). The O antigen plays important and various roles in bacteriophage interactions with the host. Many bacteriophages employ the O antigen as a primary receptor that ensures reversible adsorption to the host cell followed by irreversible adsorption to a secondary receptor, most frequently an outer membrane protein (2-4). O-antigen modifications may prevent bacteriophage binding. For instance, phage SPC35 uses the Salmonella O12 antigen receptor, and phase-variable glucosylation of the O antigen confers transient SPC35 resistance to the bacteria (5). A temperate podovirus, Sf6, also uses O antigen of its host, Shigella flexneri, as a primary receptor (4). Interestingly, the Sf6 genome harbors the oac gene for O-antigen acetylase that causes O-serotype conversion of Sf6 lysogens, which precludes bacteriophage Sf6 adsorption to these cells (6). On the other hand, the O-antigen-carrying LPS of E. coli is able to prevent the access of phages and colicins to their outer membrane protein receptors, which are otherwise sufficient for a successful attack of the cell (7). O-antigen deficiency also enhances the sensitivity of E. coli to Shiga toxin 2-converting bacteriophages (3,8). A phage T5 mutant lacking L-shaped tail fibers that recognize polymannose O antigens showed a reduced rate of adsorption to the O-antigen-producing hosts but infected O-antigen-less strains as efficiently as the wild-type phage (9, 10).These data indicate that the O-antigen layer represents an effective shield that nonspecifically protects the bacteria from interactions of bacteriophages with their cell surface receptors. In order to penetrate this shield, the phages need to acquire the proteins that specifically recognize the O antigen, thus becoming dependent on a given O-polysaccharide type. Many bacteriophages that use the O antigen as a primary receptor possess enzymes that degrade or modify it (11, 12).N4-like bacteriophage G7C and its host E. coli 4s were isolated from horse feces in the course of an investigation of coliphage ecology in the equine gut ecosystem (13). In addition to G7C, E. coli 4s was used as a host for the isolation and propagation of several other G7C-related phages (14; A. K. Golomidova, unpublished data). Currently, E. coli 4s remains the only known host for bacteriophage G7C, but despite the extremely narrow host range, G7C-related phages persisted in the same horse population for several years (14). The mechanisms that help G7C avoid extinction despite the small fraction of the total E. coli population that is suitable for its growth are poorly understood (9). Elucidation of the molecular details of the initial steps of th...