Читать книгу Principles of Virology - Jane Flint, S. Jane Flint - Страница 219

Because viruses are obligate intracellular parasites, their genome must enter a cell for the viral reproduction cycle to occur. At first sight, the physical properties of the virus particles appear as obstacles to this seemingly simple goal. Virus particles are too large to diffuse passively across the plasma membrane. Furthermore, the viral genome is encapsidated in a protective coat that shields the nucleic acid as it travels through the harsh extracellular environment. These apparent obstacles must all be overcome during the process of viral entry into cells. Infection of cells by many, but not all, viruses requires binding to a receptor molecule on the cell surface. Exceptions include virus particles of yeasts and fungi, which have no extracellular phases, and plant viruses, which are thought to enter cells in which the cell wall has been physically damaged, for example, by insects or farm machinery.

In addition to binding viral particles, cell surface receptor molecules participate in entry, a process that relies on usurpation of normal cellular processes, such as endocytosis, membrane fusion, vesicular trafficking, and transport into the nucleus. The viral genome has to be released from the interior of the virus particle, a process known as uncoating. The receptor plays a role in this process by either initiating conformational changes that prime fusion or uncoating or by directing the virus particle into endocytic pathways, where fusion and uncoating may be triggered by low pH or by the action of proteases. These steps ultimately deliver the viral genome to the site of replication, which can be the cytoplasm, for most RNA-containing viruses, or the nucleus, for most DNA-containing viruses.