Virus replication cycle
Viral life cycle passes through definite consecutive stages:
– viral attachment;
– penetration (virus entry);
– biosynthesis of viral components;
– morphogenesis with assembly of viral particles;
– virion release (or egress).
(1) The first step of infection is the attachment stage with adsorption of virions to the host cells. Viral spikes, containing attachment proteins, project out of the surface of the viral shell. Viral receptor molecules as well as the opposite cell receptors are usually glycoproteins. In some cases virus interacts with cell protein sequences (picornaviruses) and with oligosaccharides (orthomyxoviruses or paramyxoviruses).
Сell receptor density for particular virus is in the range 10,000-100,000 per infected cell.
(2) Viral penetration or virus entry involves different mechanisms, depending on virus nature. Naked viruses are captured into the host cell by endocytosis (or viropexis) after virus adsorption. Adsorption usually occurs in membrane sites enriched with cellular receptor proteins clathrin or caveolin.
Virion-containing vesicle (endosome or vacuole) is opened in cytoplasm, disseminating viral particles.
Enveloped viruses can use the endocytosis mechanism also. The fragment of the cell plasma membrane enwraps the attached virion with the vesicle formation. Further the virus lipid envelope fuses with the cell membrane due to hydrophobic forces with subsequent release of free nucleocapsids into the cytoplasm.
In another case the direct fusion of viral envelope with the plasma membrane is performed following the strong specific adsorption of viruses to the host cell receptors. Specific fusion proteins stimulate membrane fusion (e.g. F-protein of paramyxoviruses, the similar action is controlled by influenza virus hemagglutinin). Fusion proteins promote specific type of viral cytopathic activity (see below), causing host cell integration with symplasts and syncytium appearance. The envelope-devoid nucleocapsids are then liberated into the cytoplasm under the inner side of the cell membrane.
(3) Uncoating of virus results in nucleic acid release from the surrounding proteins before the genome replication and early protein synthesis. Uncoating follows viral entry and continues after penetration. Dissolution of virus is facilitated by acidic pH in the endosome. Viral genome can be liberated as a naked nucleic acid (property of picornaviruses) or as a nucleocapsid (essential for reoviruses). In latter case it carries polymerases, necessary for further viral replication.
(4) Biosynthesis of viral components varies strongly in different viruses. It depends on viral nucleic acid structure and polarity.
DNA viruses are reproduced usually in the nucleus of the infected cells. They use cell DNA and RNA polymerases for nucleic acid replication.
The majority of DNA viruses contains double-stranded DNA, which is transcribed into sense mRNA (e.g herpesviruses). The latter is used as a pattern for protein synthesis.
The final transcript of mRNA can be achieved in several ways including reading frame shift or change of transcription starting point in the same reading frame (overlapping genes).
Splicing of primary transcript, where the elimination of inserted non-coding RNA fragments occurs resulting in formation of mature mRNA, is essential for adenoviruses.
Newly formed viral mRNAs are translated on cellular ribosomes yielding viral proteins.
Virus-induced early viral proteins are synthesized before the replication of viral genome. They are produced in host cell ribosomes usung viral mRNA template. Most of them are viral enzymes and regulatory protens serving for the next steps of viral reproduction.
Late proteins are mostly the structural units of viral capsid; they are formed after the replication of viral genomic nucleic acid.
Replication patterns of RNA viruses are even more entangled.
For instance, reoviruses use initial double stranded segmented RNA for mRNA synthesis by the own viral RNA polymerase.
Positive single-stranded RNA of many viruses (e.g. picornaviruses or flaviviruses) is infectious, and it is used as a template for direct protein synthesis. Viral RNA of these viruses is multiplied through double-stranded plus-minus RNA intermediate.
Viruses, containing negative genomic RNA (e.g. rhabdoviruses, paramyxoviruses, orthomyxoviruses), synthesize positive RNA strand by viral RNA polymerase. In case of segmented viral genomes (orthomyxoviruses) mRNA is transcribed sequentially from different segments.
Long replication cycle of retroviruses is maintained by viral reverse transcriptase. It catalyzes DNA copy formation on the viral RNA template. After DNA integration into the host cell genome it is used for transcription of mRNA, coding for the viral proteins.
Viral genomic RNA is usually multiplied in the cytoplasm of infected cells with some exceptions (e.g. retroviruses).
RNA-containing viruses are characterized by the almost simultaneous expression of all viral proteins. Some viruses (picornaviruses, retroviruses) translate mRNA into common precursor polyprotein, which is cleaved by proteases with final formation of protein sequence.
Overall, viruses demonstrate the disjunctive type of reproduction, where viral components (DNA and proteins of the coat) are synthesized in separate bacterial cell compartments.
(5) Viral morphogenesis (or maturation stage) includes the self-assembly of virions within the cells. It is the multistep process of viral capsid formation and nucleic acid packing.
The time interval between the virus penetration and the end of viral assembly is known as the eclipse period, where the virus is deprived of infectivity, being incapable of causing infectious process. The infectivity is restored only after full-value maturation of virus progeny.
(6) Virion release (or viral egress) is performed in several ways. In case of the cell death due to viral infection the virus is liberated by the lysis of the host cell. Another mechanism is budding through the cellular membrane, which retains the viability of the infected cells.
For enveloped viruses the maturation step is accomplished during budding, where the fragments of plasma membrane cover the nucleocapsid making viral envelope with parallel embedding of matrix proteins and spikes.
The whole length of the virus replication cycle varies from 6-8 hours for picornaviruses to several days for adenoviruses or measles viruses.
Sometimes after reproduction cycle the defective viruses are formed, which are usually non-infectious. Such virus particles lack some important genes due to incorrect nucleic acid excision or impaired viral protein translation and assemblage.
Outcomes of viral infection result in the productive, persistent, transforming and latent infection.
Productive infection leads to active accumulation of viral particles with destruction of infected cells e.g., by lysis. Newly synthesized virions are able to spurt the infection, penetrating into neighboring susceptible cells. In most of cases it is characteristic for acute viral infections.
Persistent infection progresses much more slowly. It might be followed by low viral replication with slow budding from the infected cells. Thus, the host cells survive and can propagate. It is related with chronic viral infection
Latent virus infection evolves, when the virus is continuously present inside the infected cells, but its reproduction is very slow or even ceased.
Likewise, latent infection is established after the integration of the viral genome into the cell DNA with formation of provirus (integrative infection). In that case mature virus particles are not produced for a long time. Latent infection also corresponds to chronic viral disease.
Transforming infection is promoted by particular type of viruses (e.g. Oncovirinae subfamily representatives, papillomaviruses, etc.) Transformation is mediated by complex chain of genetic events, caused by virus, which affect the genome of susceptible cells. The virus can initiate tumor growth in the infected organism.