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 glycoprotein eventually drive the fusion of viral and cellular membranes. For nonenveloped viruses, conformational changes generally lead to disruption of the cellular membrane and the delivery of a subviral complex to the cell interior. In both cases, these changes ultimately allow the viral genome to access a cellular compartment that enables replication. Although the mechanisms of entry for the various virus families appear vastly different, certain themes repeatedly emerge. Structural rearrangements in viral proteins that enable entry are often dependent on proteolytic cleavage events that occur either during assembly in the virus-producing cell or at the surface or within endosomal compartments during entry into the target cell. The acid pH found in endosomes is a common trigger for conformational rearrangements that enable entry. Such conformational changes almost always result in the exposure of hydrophobic protein sequences that can interact with and disrupt cellular membranes and allow access to the cell interior.
The cell is often not an idle target but an active participant in viral entry. Engagement of cell surface receptors by virus particles can trigger signal transduction pathways that lead to cytoskeletal rearrangement and endocytosis. Virus particle transport within the cell can be within vesicles, whose transport mechanisms are quite well understood. Conversely, vesicle-independent transport of viral or subviral particles on the cytoskeletal network is less well characterized. Notably, entry of various components of virus particles, nucleic acids and proteins, into the interior of the cell can be detected by specialized sensors that alert the innate immune system and elicit antiviral responses (a topic covered in Volume II, Chapter 3).
For some viruses, the final destination, and the site of genome replication, is the cell’s nucleus. The nuclear envelope raises an additional barrier to virus entry, with a plethora of proteins regulating access to the nuclear interior through the nuclear pores. Virus particles or subviral structures are too large to pass through the nuclear pore. Therefore, interactions with the specialized nuclear transport machinery are usually necessary for subviral structures to be escorted into the nuclear interior. This process is not well understood for many viruses.
Many questions about specific steps in the entry pathways of many viruses remain, including the elucidation of entry pathways used in whole organisms, a technically challenging endeavor. Understanding how entry proceeds and how particles “disassemble” to release the viral genome at the site of replication will allow us not only to develop specific interventions for prevention of virus infections but also to manipulate virus particles for use as viral vectors.
REFERENCES
Books
Pohlmann S, Simmons G. 2013. Viral Entry into Host Cells. Landes Bioscience, Austin, TX.
Reviews
Cosset F-L, Lavillette D. 2011. Cell entry of enveloped viruses. Adv Genet 73:121–183.
Fay N, Panté N. 2015. Old foes, new understandings: nuclear entry of small non-enveloped DNA viruses. Curr Opin Virol 12:59–65.
Grove J, Marsh M. 2011. The cell biology of receptor-mediated virus entry. J Cell Biol 195:1071–1082.
Harrison SC. 2015. Viral membrane fusion. Virology 479-480:498–507.
Moyer CL, Nemerow GR. 2011. Viral weapons of membrane destruction: variable modes of membrane penetration by non-enveloped viruses. Curr Opin Virol 1:44–49.
Papers of Special Interest
Bullough PA, Hughson FM, Skehel JJ, Wiley DC. 1994.
