Viruses nucleocapsid release

The first spike proteins can be detected at the cell surface about 2 hours after infection (Birdwell and Strauss, 1974 K riainen et al., 1980). It takes about 1 hour more before mature viral particles are released extracellularly. The virus is released from the cell by a budding outward of the cell membrane. In this process the nucleocapsid binds to the plasma membrane which wraps around the nucleocapsid and the bud is expelled from the cell (Acheson and Tamm, 1967). 

Viral Infection. The infection cycle is initiated when the glycoprotein spikes on the virion bind to receptors on the cell surface. The virus is localized initially to coated pits, where it is engulfed in a coated vesicle and transported to the endosomal compartment within the interior of the cell. A decrease in the pH in the interior of the vesicle induces a conformational change in the glycoprotein spikes, and rearrangement of the El glycoprotein mediates fusion of the virion envelope with the endosomal membrane.76 This fusion results in the release of the nucleocapsid into the cytoplasm, where disassembly of the nucleocapsid releases the viral RNA genome to the synthetic apparatus of the cell. 

Arterivirus (equine arteritis virus and simian hemorrhagic fever virus). Enveloped, polyhedral, positive-sense, and ssRNA. Synthesis occurs in the host cell cytoplasm maturation involves budding of nucleocapsids through the host cell plasma membrane. Viruses are released via cell lysis (Arterivirus). Many replicates in arthropods and vertebrates. 

One would assume that the mechanism for delivery of the nucleocapsid through the membrane of the intracellular vacuole has to be provided by the virus. There are no known precedents in normal cell physiology for the passage of macromolecular assemblies like the viral nucleocapsids into the cytoplasm. The most likely mechanism would be fusion of the viral envelope with the vacuolar membrane and subsequent release of the nucleocapsid into the cytoplasm. But if penetration occurs by fusion why would this occur intracellularly and not at the cell surface The clue comes from the low pH dependence of the infection. 

Analysis of SFV entry has thus shown that the virus binds to receptors on the cell surface and moves by lateral diffusion into coated pits to be internalized by coated vesicles. The endocytosed virus is delivered into endosomes. Here presumably, the viral envelope is activated by the low pH prevailing in this compartment to fuse with the vacuolar membrane. This results in the release of the viral nucleocapsid into the cytoplasm. During normal infection, the virus might not enter into lysosomes although SFV particles have been identified in this compartment using the large loads of virus needed to visualize the entry process by electron microscopy. Even if this were to happen normally, the viral nucleocapsid would escape destruction because of the rapidity of the fusion mechanism. 

The infection cycle is shown in Figure 19.2. When the polyhedra are dispersed in the environment, the viral particles inside them (ODVs) are deposited on the plant leaves. When the caterpillars feed on the virus-contaminated foliage, they ingest the polyhedra. The alkaline environment in the caterpillar medium intestine causes breakdown of the polyhedra and the viral particles are released from the polyhedra. The infection of the cells occurs via a receptor-mediated fusion process. Once in the cytoplasm, the nucleocapsids without membrane are transported to the nucleus of the cell, where gene expression and genome replication begins. 

Unfortunately, there are no structures available for either the flaviviruses or alphaviruses under conditions approximating the fusion state. For both groups of viruses, entry is believed to occur following attachment of the virus to the cellular receptor and internalization of the particle into an endosome (Kielian, 1995 Heinz and Allison, 2001). Acidification of the endosome results in rearrangement of envelope proteins and subsequent insertion of the fusion peptide into the endosomal membrane (Levy-Mintz and Kielian, 1991 Allison et al., 2001). Ultimately this results in fusion of cellular and viral membranes and release of the nucleocapsid core and genome RNA into the cytoplasm of the infected cell. In vitro experiments... 

Fig. 43.2. Replicative cycle of HIV. (1) The virus gp120 protein binds to CD4 resulting in fusion of the viral envelope and the cellular membrane and the release of viral nucleocapsid into the cytoplasm. (2) The nucleocapsid is uncoated, and viral RNA is reverse transcribed by reverse transcriptase (RT). (3) The resulting double-stranded proviral DMA migrates into the cell nucleus and is integrated into the cellular DMA by integrase (IN). (4) Proviral DNA is transcribed by the cellular RNA polymerase II. (5) The mRNAs are translated by the cellular polysomes. (6) Viral proteins and genomic RNA are transported to the cellular membrane and assemble. Immature virions are released, and polypeptide precursors are processed by the viral protease (PR) to produce mature vital particles. (Adapted from Sierra S, Kupfer B, Kaiser R. Basics of the virology of HIV-1 and its replication. J Clin Virol 2005 34 233 with permission from Elsevier.)...

Within the host cell, the vims nucleic acid is released from both the nucleocapsid and the membrane envelope (if present). This process, known as uncoating, involves, inter alia, the action of specific proteases. The duration of uncoating increases with the complexity of the virus and with the tightness of the packing of the nucleic acid. Uncoating of small V., e.g. picomaviruses is complete in about 2 h., whereas the uncoating of poxviruses lasts 10-12 h. 

The stepwise dissociation of the Semliki Forest (SF) Viral membrane with Triton X-100 [82] has been described. The SF virus has one of the simplest biological membranes it has a spherical nucleocapsid consisting of RNA and one polypeptide. The nucleocapsid is surrounded by a lipid-protein membrane similar in composition to that of the host cell plasma membrane as it acquires this protective layer by budding as it leaves the cell. Binding of Triton to the membrane occurs below the CMC and increases with increasing surfactant concentration. Release of the nucleocapsids from the virus occurs when more than 0.2 to 0.4 mg Triton is bound per mg membrane. When higher concentrations of Triton were present (c 1.6 mg bound) the membranes dissociate to protein-lipid-surfactant complexes, followed at higher surfactant concentrations by delipidation of membrane protein (see Fig. 10.13). Lipophilic proteins isolated... 

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