Principles of Virology. Jane Flint
acid is injected into the nucleus through a portal in the nucleocapsid (Fig. 5.26B). The DNA of some bacteriophages is packaged in virus particles at high pressure, which provides sufficient force to insert the viral DNA genome into the bacterial cell (Box 5.6). A similar mechanism may allow injection of herpesviral DNA. Herpesvirus capsids also dock onto the nuclear pore complex, and interaction with nucleoporins destabilizes a viral protein, pUL25, which locks the genome inside the capsid. This event drives the naked viral DNA, which is packaged in the nucleocapsid under very high pressure, to exit through the portal. Ordinarily, the charged, hydrophilic viral nucleic acid would have difficulty passing through the pore, but this mechanism overcomes the requirement for hydrophobic interactions with nucleoporins.
In contrast to herpesvirus particles, partially disassembled adenovirus capsids dock onto the nuclear pore complex by interaction with NUP214 (Fig. 5.26C and 5.27). Release of the viral genome requires capsid protein binding to kinesin-1, the motor protein that mediates transport on microtubules from the nucleus to the cell periphery. As the capsid is held on the nuclear pore, movement of kinesin-1 toward the plasma membrane is thought to pull the capsid apart (Fig. 5.27). The released protein VII-associated viral DNA is then imported into the nucleus, where viral transcription begins.
The 26-nm capsid of parvoviruses is small enough to fit through the nuclear pore (39 nm), and it has been assumed that these virus particles enter by this route. However, there is no experimental evidence that parvovirus capsids pass intact through the nuclear pore. Instead, virus particles bind to the nuclear pore complex, followed by disruption of the nuclear envelope and the nuclear lamina, leading to entry of virus particles (Fig. 5.26D). After release from the endoplasmic reticulum, the 45-nm capsid of simian virus 40 also docks onto the nuclear pore, initiating disruption of the nuclear envelope and lamina. Such nuclear disruption appears to require cell proteins that also participate in the increased nuclear permeability that takes place during mitosis, raising the possibility that nuclear entry of these viral genomes is a consequence of remodeling a cellular process.
Import of Retroviral Genomes
Fusion of the viral membranes of most retroviruses with the plasma membrane releases the viral core into the cytoplasm. The retroviral core consists of the viral RNA genome, coated with NC protein, and the enzymes reverse transcriptase (RT) and integrase (IN), enclosed in a shell comprising the capsid (CA) protein. The RNA is reverse transcribed into DNA, which has to reach the nucleus in order to integrate and replicate as part of the host genome (see Chapter 7). The capsid core surrounding the viral RNA allows nucleotides necessary for reverse transcription to enter, but not larger molecules. It is thought that this core has to at least partially disassemble for DNA synthesis to continue but does not completely dissociate from the preintegration complex, comprising the viral DNA, IN, and other proteins. The mechanism of nuclear import of the preintegration complex is poorly understood, but it is quite clear that this structure is too large to pass through the nuclear pore complex. The betaretrovirus Moloney murine leukemia virus can efficiently infect only dividing cells when the nuclear membrane breaks down during mitosis. The viral preintegration complex has to then be tethered to chromatin so that it remains associated with cellular DNA when the nuclear membrane re-forms in daughter cells, circum-venting the need for active transport.
DISCUSSION
The bacteriophage DNA injection machine
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).
Perspectives
As the Trojans brought the vehicle of their destruction into their city, so do cellular processes bring viral particles inside the cell. Although the initial encounter between a single virus particle and a cell is random, viral proteins often exploit specific cell surface molecules to secure specific docking to their target cells. A diverse set of cell surface molecules are found to serve as viral receptors. On the other hand, the same molecule or molecules belonging to the same family of proteins can serve as receptors for divergent viruses.
Receptor binding is but the first step, and often initiates major conformational changes in the virus particles. For enveloped viruses, such conformational changes in the envelope