Читать книгу Principles of Virology - Jane Flint, S. Jane Flint - Страница 255

The mechanisms by which the bacteriophage genome enters the bacterial host are unlike those for viruses of eukaryotic cells. One major difference is that the bacteriophage particle remains on the surface of the bacterium as the nucleic acid passes into the cell. The DNA genome of some bacteriophages is packaged under high pressure (up to 870 lb/in²) in the capsid and is injected into the cell. The complete structure of bacteriophage T4 illustrates this remarkable process. To initiate infection, the tail fibers attach to receptors on the surface of Escherichia coli. Binding induces a conformational change in the baseplate, which leads to contraction of the sheath. This movement drives the rigid tail tube through the outer membrane, using a needle at the tip. When the needle touches the peptidoglycan layer in the periplasm, the needle dissolves and three lysozyme domains in the baseplate are activated. These enzymes disrupt the peptidoglycan layer of the bacterium, allowing DNA to enter.

 Browning C, Shneider MM, Bowman VD, Schwarzer D, Leiman PG. 2012. Phage pierces the host cell membrane with the iron-loaded spike. Structure 20:326–339.

Structure of bacteriophages and membrane-piercing spike. (A) A model of the 2,000-Å bacteriophage T4 as produced from electron microscopy and X-ray crystallography. Components of the virion are color coded: head (beige), tail tube (pink), contractile sheath around the tail tube (green), baseplate (multicolored), and tail fibers (white and magenta). In the illustration, the virus particle contacts the cell surface, and the tail sheath is contracted prior to DNA release into the cell. Courtesy of Michael Rossmann, Purdue University. (B) Cryo-electron microscopic reconstruction of phi92 baseplate. The spike is shown in red. (C, D) Trimers of bacteriophage phi92 gp138, shown as surface (C) and ribbon diagrams (D).

Figure 5.27 Uncoating of adenovirus at the nuclear pore complex. After release from the endosome, the partially disassembled capsid moves toward the nucleus by dynein-driven transport on microtubules. The particle docks onto the nuclear pore complex protein NUP214 (yellow). The capsid also binds kinesin-1 light chains, which move away from the nucleus, pulling the capsid apart. The viral DNA, bound to protein VII, is delivered into the nucleus by the import protein transportin and other nuclear import proteins (not shown).

In contrast to Moloney murine leukemia virus, other retroviruses, such as lentiviruses, can reproduce in nondividing cells. The preintegration complex of these viruses must therefore be transported through the nuclear pore by a mechanism that remains unclear. For human immunodeficiency virus type 1, increasing evidence suggests that CA-mediated attachment of the preintegration complex to NUPs is required for nuclear import. NUP engagement appears flexible, and several other capsid-interacting proteins affect the use of particular NUPs. At least some capsid proteins are transported into the nucleus with the preintegration complex, and their interaction with nuclear proteins influences integration site selection (see Chapter 10).