Bactériophage

Les bactériophages, ou phages (mot formé des éléments bactério-, « bactérie », et -phage, « qui mange »), ou, plus rarement, virus bactériens, sont des virus qui n'infectent que des bactéries. Ils sont présents dans toute la biosphère. Ils sont particulièrement abondants dans les milieux riches en bactéries, et donc notamment dans les excréments, le sol et les eaux d'égout. Dans un millilitre d'eau de mer, on compte près de 50 millions de bactériophages. Le support de l'information génétique (génome) des bactériophages peut être un ADN ou un ARN. Parce que leur génome est entouré d’une capside, les phages font partie des virus dits complexes. L'activité des bactériophages est découverte en 1897 par le franco-canadien Félix d'Hérelle lorsqu'il remarque des trous dans les cultures de bactéries qu’il développe pour lutter contre les essaims de sauterelles en Amérique centrale. Il n’en comprend le sens qu'en 1917, lorsqu'il fait la même observation dans des selles de malades atteints

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Structure and Function of the T4 Spackle Protein Gp61.3

Alec Fraser, Petr Leiman

The bacteriophage T4 genome contains two genes that code for proteins with lysozyme activity-e and 5. Gene e encodes the well-known T4 lysozyme (commonly called T4L) that functions to break the peptidoglycan layer late in the infection cycle, which is required for liberating newly assembled phage progeny. Gene product 5 (gp5) is the tail-associated lysozyme, a component of the phage particle. It forms a spike at the tip of the tail tube and functions to pierce the outer membrane of the Escherichia coli host cell after the phage has attached to the cell surface. Gp5 contains a T4L-like lysozyme domain that locally digests the peptidoglycan layer upon infection. The T4 Spackle protein (encoded by gene 61.3) has been thought to play a role in the inhibition of gp5 lysozyme activity and, as a consequence, in making cells infected by bacteriophage T4 resistant to later infection by T4 and closely related phages. Here we show that (1) gp61.3 is secreted into the periplasm where its N-terminal periplasm-targeting peptide is cleaved off; (2) gp61.3 forms a 1:1 complex with the lysozyme domain of gp5 (gp5Lys); (3) gp61.3 selectively inhibits the activity of gp5, but not that of T4L; (4) overexpression of gp5 causes cell lysis. We also report a crystal structure of the gp61.3-gp5Lys complex that demonstrates that unlike other known lysozyme inhibitors, gp61.3 does not interact with the active site cleft. Instead, it forms a "wall" that blocks access of an extended polysaccharide substrate to the cleft and, possibly, locks the enzyme in an "open-jaw"-like conformation making catalysis impossible.

MDPI2020

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Phage Pierces the Host Cell Membrane with the Iron-Loaded Spike

Christopher Browning, Petr Leiman

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 beta-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 phi 92 and identify the residues that constitute the membrane-attacking apex. Both proteins form compact spikes with a similar to 10 angstrom 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.

2012

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Branched Lateral Tail Fiber Organization in T5-Like Bacteriophages DT57C and DT571/2 is Revealed by Genetic and Functional Analysis

Ricardo Cesar Guerrero Ferreira, Nikolai Prokhorov

The T5-like siphoviruses DT57C and DT571/2, isolated from horse feces, are very closely related to each other, and most of their structural proteins are also nearly identical to T5 phage. Their LTFs (L-shaped tail fibers), however, are composed of two proteins, LtfA and LtfB, instead of the single Ltf of bacteriophage T5. In silico and mutant analysis suggests a possible branched structure of DT57C and DT571/2 LTFs, where the LtfB protein is connected to the phage tail via the LtfA protein and with both proteins carrying receptor recognition domains. Such adhesin arrangement has not been previously recognized in siphoviruses. The LtfA proteins of our phages are found to recognize different host O-antigen types: E. coli O22-like for DT57C phage and E. coli O87 for DT571/2. LtfB proteins are identical in both phages and recognize another host receptor, most probably lipopolysaccharide (LPS) of E. coli O81 type. In these two bacteriophages, LTF function is essential to penetrate the shield of the host's O-antigens. We also demonstrate that LTF-mediated adsorption becomes superfluous when the non-specific cell protection by O-antigen is missing, allowing the phages to bind directly to their common secondary receptor, the outer membrane protein BtuB. The LTF independent adsorption was also demonstrated on an O22-like host mutant missing O-antigen O-acetylation, thus showing the biological value of this O-antigen modification for cell protection against phages.

Mdpi Ag2016

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