Virus Interactions With the Cell

Virus replication requires specific interactions with host cells. The replication cycle begins with attachment of viral proteins to host cell receptors. The presence or absence of receptors is an important factor in determining if the cell is permissive for infection. The next step in the virus replication cycle is transfer of the genome into cytosol or nucleoplasm. Some viruses transport just their nucleic acid genomes into the cell while others deliver the entire virion. Once in the cell, virion proteins and genome interact with a variety of cell proteins, nucleic acids, and membranes. Productive replication requires synthesis of viral mRNAs, protein, and genomes. The details of these processes vary widely. However, to be successful a virus must be able to compete with host cell for building materials. For example, as the cell is constantly synthesizing proteins, viral mRNAs must be able to redirect ribosomes to their own mRNAs. Some viruses can shut down cellular transcription and translation to redirect those processes to the production of viral proteins. In contrast DNA viruses have developed methods to induce cell DNA replication and/or cell division to obtain materials necessary for genome synthesis. Viruses pack a lot of information into their relatively small genomes. Most do not have the complex promoters that drive cell transcription nor do they have long noncoding introns in their genes. Viruses that replicate in the nucleus often use alternative splicing to generate families of related mRNAs from a single precursor transcript. In other cases a single transcript may be used to produce multiple proteins by ribosome-mediated processes such as leaky scanning, stop codon suppression, and frame shifting. Once viral building blocks have been synthesized, new virions are assembled and must leave the cell. Again, the details of these processes can vary widely. Some of the simplest viruses can assemble in a test tube, from purified capsid proteins and genomes. More complex viruses use a variety of cell proteins and structures for assembly and release.