Uncoating, viral particles

Viral replication consists of several steps (Figure 49-1) (1) attachment of the vims to receptors on the host cell surface (2) entry of the virus through the host cell membrane (3) uncoating of viral nucleic acid (4) synthesis of early regulatory proteins, eg, nucleic acid polymerases (5) synthesis of new viral RNA or DNA (6) synthesis of late, structural proteins (7) assembly (maturation) of viral particles and (8) release from the cell. Antiviral agents can potentially target any of these steps. 

The retrovirus enters the cell through the CD4 molecule on the cell surface. Once inside, the virus is uncoated with the help of the reverse transcriptase enzyme, enabling the virus single-stranded RNA to be converted into DNA. The viral DNA migrates to the nucleus of the cell, where it is spliced into the host DNA with the help of the integrase enzyme. Once combined, the HIV DNA is called the provirus and is duplicated each time the cell divides. The protease enzyme assists in the assembly of a new form of the viral particles. 

Amantadine has its primary medical use as an antiviral drug in the treatment of influenza by virtue of its ability to block a viral ion channel that is required for uncoating of the viral particle. In addition, it possesses weak channel blocking activity at NMDA receptors. A double-blind trial of amantadine (2.5 mg/kg for 1 week,... 

Once recognition and subsequent tight binding have occurred, the virus must penetrate the cell membrane of the host cell. Those viruses that have an outer envelope become engulfed by the cell membrane in a process called pinocytosis. The viral envelope becomes fused with the cell membrane and ultimately this breaks open to release the virion into the cytoplasm of the host cell. Its coat protein is then digested and the genome (DNA or RNA) is revealed. Viruses that do not possess an outer envelope also associate with the host cell membrane but in this case a pore opens up and the viral particle passes through it into the cytoplasm (endocytosis), where it is uncoated. 

The mechanisms by which ICAM-1 binding to HRV triggers virus destabilization and uncoating are not understood. A hydrophobic pocket inside VPl lies directly beneath the canyon floor. This pocket was shown to be the binding site for certain antiviral compounds that inhibit the replication of HRVs and related picornaviruses (Fox et al., 1986 Smith et al., 1986). Experimental evidence has shown that antiviral drugs bound to this hydrophobic pocket lock the viral particle in a state in which neither VP4 nor the N-terminus of VPl can be externalized (Lewis et al., 1998), thus preventing uncoating. 

Fig. 14.22. Infection of a host cell by HIV. The HIV virus particle binds to the CD4 receptor and a chemokine coreceptor in the host cell membrane. The virus enters the ceU and uncoats, releasing its RNA and proteins. The viral enzyme reverse transcriptase produces a double-stranded DNA copy that is integrated into the host cell genome. HIV is now a provirus. Transcripts of the viral DNA are sphced and translated to produce the proteins Tat, Rev, and Nef. Tat stimulates transcription of the viral DNA, and Rev causes the viral RNA transcripts to leave the nucleus unsphced. The unspliced RNA serves as the viral genome and also codes for the proteins of the viral core and envelope. The envelope proteins (gp41 and gpl20, which are derived from the env protein) enter the cell membrane. The viral core proteins are S5mthesized as a polyprotein, which is cleaved by a protease as the viral particles form and bud from the ceU membrane. The particles carry membrane lipid as a coat that contains gp41 and gpl20. Nef indirectly aids in the assembly of viral particles. Pol is the reverse transcriptase produced from the viral RNA. = stimulates.

Amantadine hydrochloride (1-adamantanamine hydrochloride) is a symmetric, tricyclic, primary amine that inhibits penetration of RNA viral particles into the host cell (19). It also inhibits the early stages of viral replication by blocking the uncoating of the viral genome and the transfer of nucleic acid into the host... 

Pleconaril is a broad spectrum anti-picoma virus agent [34], It is a small cyclic drug which binds to a canyon pore of the virus. In doing so it blocks the attachment and uncoating of the viral particle [35,36],... 

Reoviruses enter cells within phagocytic vacuoles and remain within these vacuoles until they fuse with primary or secondary ly-sosomes (Dales et al., 1965). Within lysosomes, reovirions are uncoated and the virion transcriptase is activated. The formation of nascent virions, the synthesis of ssRNAs and dsRNAs, and the maturation of complete virions occur within regions of the cytoplasm known as viral factories. The intracytoplasmic route by which sub-viral particles move to these sites of replication is not clear. Furthermore, the mechanism of formation of viral factories is not understood. 

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. 

Although the specific details of replication vary among types of viruses, the overall process can be described as consisting of five phases (1) attachment and penetration, (2) uncoating, (3) synthesis of viral components, (4) assembly of virus particles, and (5) release of the... 

During uncoating several different rhinovirus subparticles are observed. These are thought to be intermediates in the uncoating process [32-33]. The fully infectious 149S particle becomes an 125S A-particle, which has lost VP4 but still maintains viral RNA. This A-particle is no longer infectious. The A-particle then releases RNA to become an 80S empty shell. 

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.)...

The whole virus or its genetic material alone (DNA or RNA) enters the cell s cytoplasm (penetration and uncoating). A virus may have different penetration mechanisms in the host cell. For enveloped virus, fusion of membrane sometimes occurs. Most viruses are introduced into the cell by a kind of phagocytosis named viropexis. Virus particles are transported along the network of cytoplasmic microtubules to a specific cell site where subsequent replication takes place. Uncoating results in the liberation of viral nucleic acids into the cell which makes them sensitive to nucleases. 

Particles uncoating, release and transport of viral nucleic acid and core proteins. 

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