Capsid enveloped

Interesting observations have also been made by Shao and Zhang [31], who used cryo-AFM to investigate the effect of osmotic stress on the influenza virus, where the virus was first adsorbed onto mica, where excess particles were removed. The remaining surface particles were then immersed in deionised water, producing osmotic shock, which caused the capsid envelope to burst and release RNA. Here the rupture pore was also clearly visible, allowing the internal structure to be analysed. 

In some viruses, the capsid is surrounded by a lipid membrane (envelope), which is derived from the host cell membrane at the site of vims budding. The membrane contains viral envelope glycoproteins as well as host cell membrane proteins. 

Helical symmetry was thought at one time to exist only in plant viruses. It is now known, however, to occur in a number of animal virus particles. The influenza and mumps viruses, for example, which were first seen in early electron micrographs as roughly spherical particles, have now been observed as enveloped particles within the envelope, the capsids themselves are helically symmetrical and appear similar to the rods of TMV, except that they are more flexible and are wound like coils of rope in the centre of the particle. 

Paramyxoviruses Mumps virus Enveloped particles variable in size, 110-170nm in diameter, with helical capsids Infection in children produces characteristic swelling of parotid and submaxillary salivary glands. The disease can have neurological complications, e.g. meningitis, especially in adults... 

Measles virus Enveloped particles variable in size, 120-250nm in diameter, helical capsids Very common childhood fever, immunity is life-long and second attacks are very rare... 

Togaviruses Rubella Spherical particles 70 nm in diameter, a tightly adherent envelope surrounds an icosahedral capsid Causes German measles in children. An infection contracted in the early stages of pregnancy can induce severe multiple congenital abnormalities, e.g. deafness, blindness, heart disease and mental retardation... 

The non-enveloped human viruses all have icosahedral capsids. The structural proteins undergo a self-assembly process to form capsids into which the viral nucleic acid is packaged. Most non-enveloped viruses accumulate within the cytoplasm or nucleus and are only released when the cell lyses. 

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. 

An additional virus that has more recently gained some attention as a possible vector is that of the sindbis virus. A member of the alphavirus family, this ssRNA virus can infect a broad range of both insect and vertebrate cells. The mature virion particles consist of the RNA genome com-plexed with a capsid protein C. This, in turn, is enveloped by a lipid bilayer in which two additional viral proteins (El and E2) are embedded. The E2 polypeptide appears to mediate viral binding to the surface receptors of susceptible cells. The major mammalian cell surface receptor it targets appears to be the highly conserved, widely distributed laminin receptor. 

Penetration. After fusion of viral and host membranes, or uptake into a phagosome, the virus particle is carried into the cytoplasm across the plasma membrane. This penetration process is an active one that requires expenditure of energy by the cell. At this stage the envelope and the capsid are shed, and the viral nucleic acids are released. The uncoating of virus accounts for the drop in infectious virus assayed, because the uncoated virus cannot withstand the assay conditions. 

HRVs are non-enveloped viruses of icosahedral overall shape [44]. Located on the exterior of the viral capsid are three structural proteins (VPl, VP2 and VP3), each consisting of an eight-stranded antiparallel -barrel. VP4 is found at the interface with the RNA inside the virus. A pocket factor is usually bound to a hydrophobic canyon binding site within the VPl -barrel. This lipid-like molecule is important for the stability of the capsid and has been... 

Bluetongue virus, the prototype virus of the Orbivirus genus in the Reoviridae family, is an non-enveloped virus with seven structural proteins (VP1-VP7) [12]. The outer capsid consists of two proteins, VP2 and VPS. The core exhibits icosahedral symmetry and is composed of five proteins, two major (VPS and VP7) and three minor (VP1,VP4 and VP6) classified according to... 

Viruses are composed of one or more strands of a nucleic acid (core) enclosed by a protein coat (capsid). Many viruses possess an outer envelope of protein or... 

Unlike in bacteria and fungi, viruses do not have a protective coat that separates essential proteins and nucleic acids from the environment. The majority of viruses consist of nucleic acid polymers (DNA or RNA) enclosed within a protein coat (capsid). Sometimes, viruses pick up a lipid membrane (envelope) from the host cell that surroimds the capsid. The average size of viral particles is in the range 10-300 nm. The most common... 

Assembly and release. The assembly of the capsid and its association with nucleic acid is then followed by release of the virus front the cell. This may occur in different ways, depending upon the nature of the virus. Naked viruses may be released slowly and extruded without cell lysis, or released rapidly by disruption of the cell membrane. DNA viruses, which mature in the nucleus, tend to accumulate within infected cells over a long period. Enveloped viruses generally acquire their envelope and leave the cell by budding through the nuclear or cytoplasmic membrane at a point where virus-specified proteins have been inserted. The budding process is compatible with cell survival. 

Viruses are complex particles, entering the cells by fusion of their envelope to the plasma membrane or by endocytosis followed by the escape of the capsid by membrane fusion or lysis (Sodeik, 2000). The diameter of the viral particle could be several hundred nanometers, implying a very inefficient diffusional movement in the cytoplasm, based on those physicochemical considerations that were discussed above (Kasamatsu and Nakanishi, 1998). Despite these limitations, those viruses that replicate in the nucleus have evolved sophisticated mechanisms to ensure a highly efficient nuclear delivery of their genetic material. Since these mechanisms may provide a conceptual framework to design novel non-viral delivery systems, we shall review some of the key elements that account for the nuclear targeting of certain viruses. 

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