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Infectious Diseases: A Shared Syringe

SBKB [doi:10.1038/sbkb.2012.138]
Featured Article - May 2013
Short description: Tail sheath proteins of one viral family share a common fold and conformational changes despite low sequence identity.

Cryo-EM reconstructions of the phiKZ tail sheath in extended (left) and contracted (right) conformations. The densities are colored radially from cyan (inside) to magenta (outside). Figure courtesy of Anastasia Aksyuk.

A crucial step in viral infection is the introduction of the viral genome into the host cell. Bacteriophages of the Myoviridae family feature an icosahedral head and a contractile tail. The infection process begins with the attachment of the tail to the bacterial outer membrane, followed by a conformational change that results in the contraction of the tail sheath proteins. Aided by a virally coded lysozyme to digest the peptidoglycan layer, this contraction forces an internal tail tube through the bacterial cell wall, much like the needle of a syringe, and allows the phage DNA to enter the cell.

Although this mechanism is well established, the details of the conformational changes occurring within the tail sheath have only been described for a single member of the Myoviridae family, bacteriophage T4. To better understand these structural rearrangements, Rossmann, Aksyuk, Tong and colleagues (PSI NESG) determined crystal structures of the sheath proteins from three Myoviridae family members: a protease-resistant core of gene product 29 (gp29PR) from bacteriophage phiKZ (PDB 3SPE), and the full-length sheath proteins from the prophages of Listeria innocua and Desulfito bacterium hafniense (PDB 3LML and 3HXL, respectively).

The structures of the two prophage sheath proteins share a common three-domain architecture, and are topologically similar to the T4 sheath protein, despite a low sequence identity with the latter (16–17%). The structure of gp29PR, which has only 11% sequence identity to T4 sheath proteins, maintains the seven-stranded β sandwich fold seen in the other proteins.

Although the authors were unable to crystallize full-length gp29, they were successful in forming sheaths of the protein in both extended and contracted conformations. By docking the crystal structure of gp29PR into cryo-electron microscopy reconstructions of these sheaths, the gp29 subunits were shown to form six-membered rings that stack on top of each other to form the long phiKZ tail in the extended state. In the contracted state, the six subunits of one ring are interdigitated into the ring above, thereby decreasing the axial rise of the sheath from 36.7 Å to 17.9 Å. These results are comparable to those of the T4 contractile sheath, and demonstrate the structural and functional conservation among Myoviridae family member proteins, despite their divergence in primary sequence.

Timothy Silverstein

References

  1. Aksyuk et al. Structural conservation of the Myoviridae phage tail sheath protein fold.
    Structure. 19, 1885-1894 (2011). doi:10.1016/j.str.2011.09.012

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Funded by a grant from the National Institute of General Medical Sciences of the National Institutes of Health