Nucleocapsid, viral

Some viruses also have a lipid membrane, called a viral envelope, surrounding the nucleocapsid. The envelope is derived from the host-cell plasma membrane. Viruses have been classified based on both the type of viral-particle packaging (nucleocapsid, viral envelope, etc.), and on the nucleic-acid composition (RNA or DNA) of the viral genome. 

All enveloped human vimses acquire their phospholipid coating by budding through cellular membranes. The maturation and release of enveloped influenza particles is illustrated in Fig. 3.8. The capsid protein subunits are transported flom the ribosomes to the nucleus, where they combine with new viral RNA molecules and are assembled into the helical capsids. The haemagglutinin and neuraminidase proteins that project fiom the envelope of the normal particles migrate to the cytoplasmic membrane where they displace the normal cell membrane proteins. The assembled nucleocapsids finally pass out from the nucleus, and as they impinge on the altered cytoplasmic membrane they cause it to bulge and bud off completed enveloped particles flxm the cell. Vims particles are released in this way over a period of hours before the cell eventually dies. 

Figure 5.29 Uptake of an enveloped virus particle by an animal cell, (a) The process by which the viral nucleocapsid is separated from its envelope, (b) Electron micrograph of adenovinis panicles entering a cell. Each panicle is about 70 nm in diameter.

If the pH is lowered to about 6.0 the SFV particle undergoes a dramatic decrease in diameter of about 70 A which is due to the contraction of the nucleocapsid (Sfiderlund et al, 1972 von Bonsdorff, 1973). The viral membrane apparently adheres to the nucleocapsid during the contraction, and excess membrane is extruded in the form of blebs. Interestingly, few spike proteins are seen on these blebs suggesting that they contain only lipid and that the spike proteins remain bound to the nucleocapsid during shrinkage. 

Binding to the cell surface proceeds at 0°C, but the cells are not infected (Helenius et al., 1980). When the cells are warmed to 37 C the virus is rapidly removed from the cell surface and infection ensues. In general there are two ways to envisage the entry of enveloped viruses into cells—either by penetration directly through the plasma membrane, or by endocytosis (engulfment by a plasma membrane-derived vesicle) (see Lonberg-Holm and Philipson, 1974). In both cases delivery of the nucleocapsid with the RNA would have to involve a fusion reaction between the viral envelope and either the cell surface membrane or the vesicle membrane. Paramyxoviruses are known to fuse their envelopes with the plasma membrane (see Hosaka and Shimizu, 1977). However, whether this process leads to productive infection has not yet been settled. 

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. 

Wengler et al. (1982) were able to assemble nucleocapsids of the correct density and size from 42 S RNA and from isolated capsid proteins. Surprisingly, the interaction between the nucleic acid and the capsid protein was found to be fairly unspecific since it was possible to substitute the viral RNA with RNA and DNA molecules ranging in size from... 

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). 

Budding is probably initiated by the viral nucleocapsid binding to a cluster of spike proteins at the cell surface. The binding must be mediated by the cytoplasmic domain of the E2 protein attaching to a capsid... 

The tobacco mosaic virus (center right), a plant pathogen, has a structure similar to that of MB, but contains ssRNA instead of DNA. The poliovirus, which causes poliomyelitis, is also an RNA virus. In the influenza virus, the pathogen that causes viral flu, the nucleocapsid is additionally surrounded by a coat derived from the plasma membrane of the host cell (C). The coat carries viral proteins that are involved in the infection process. 

During infection (1), the virus s coating membrane fuses with the target cell s plasma membrane, and the core of the nucleocapsid enters the cytoplasm (2). In the cytoplasm, the viral RNA is initially transcribed into an RNA/DNA hybrid (3) and then into dsDNA (4). Both of these reactions are catalyzed by reverse transcriptase, an enzyme deriving from the virus. The dsDNA formed is integrated into the host cell genome (5), where it can remain in an inactive state for a long time. 

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