The first stage of the virus is to attach to the cell. This is achieved by the interaction of receptors located on the cell membrane of the host cell and the virus particle. Also animal viruses only infect certain organisms and certain tissues within a host. Usually the receptors on the host cell will join to hormones or other molecules required for the cell to function properly. Most of the receptors are from a group called glycoproteins and are involved in the immune system.
Animal cells membranes have micro-domains called lipid rafts that are involved in virus entry and assembly. E.g. the receptors for enveloped viruses are concentrated in lipid rafts. When the virus binds to these receptors, the host cell is tricked and the virus is endocytosed.
The surface site of the virus that comes into contact with receptors of the host cell may simply consist of a caspid structure. In some viruses the binding site is at the bottom of the surface.
Once the virus has been adsorbed it can then penetrate the cell membrane and enter the host cell. Soon after the viral; nucleic acid prepares for replication. Before this can occur, some viruses have shed the caspid protein. – called Uncoating. The mechanism of penetration and Uncoating vary depending on the virus. Enveloped viruses may enter the host cell in a different way than a naked virus. Some viruses only inject their nucleic acid, while others make sure the RNA/DNA polymerase also enter the host cell. Virus penetration may occur in two modes:
(1) Fusion of the viral envelope with the host cell membrane or (2) entry by endocytosis
(1) The envelopes of some viruses fuse directly with the host cell plasma membrane which may involve glycoproteins that bind to the plasma membrane proteins. After attachment, a number of things happen: the membrane lipids rearrange, the adjacent halves of the contacting membranes merge and a pore is formed. The nucleocaspid enters the cell cytoplasm.
(2) non-enveloped viruses and some enveloped viruses enter the cell via endocytosis. They may be engulfed by receptor mediated endocytosis to form coated vesicles. The virions attach to clathrin-coated pits; these pits pinch off to form the vesicles containing the virus. These vesicles then fuse with endosomes after the clathrin has been removed. Depending on the virus the nucleocaspid may escape before or after the fusion. Endosomal enzymes are involved in the Uncoating process of the virus and low pH also aids in this process. The viral envelope may fuse with the Endosomal membrane and the nucleocaspid may be released into the cytoplasm. Once in the cytoplasm, the viral nucleic acid may be released (the caspid may be completely uncoated or may still function while attached to the caspid. Naked viruses lack an envelope so cannot go through the fusion process. Instead the caspid has a conformational change. The altered caspid contacts the vesicle membrane which will either release the nucleic acid through a pore or the membrane may rupture.
Replication and Transcription in DNA Viruses- the early part of synthesis is controlled by early genes, their role once the host cell has been taken over is to synthesise the viral DNA and RNA. Some animal viruses inhibit DNA, RNA and protein synthesis of the host cell. Some viruses stimulate the synthesis of macromolecules in the host cell.DNA replication usually occurs within the host cell nucleus (poxvirus –in cytoplasm). Messenger RNBA (early mRNA) is transcribed from DNA by host enzymes (poxvirus use viral polymerase).
Below are illustrated the modes of replication of few viruses.
Replication in RNA viruses
RNA viruses can be placed into four groups based on certain characteristics: positive ssRNA viruses, Negative ssRNA viruses, dsRNA viruses & Retroviruses. One aspect is common in all groups which is they have to encode an RNA-dependent RNA polymerase. This is used to synthesise mRNA or to replicate the RNA genome. Some ssRNA can carry out both functions.
An example of an RNA virus is Influenza.
Influenza virus is an envelope, single stranded (-) RNA virus that contains a segmented genome.
1. The virus adsorbs to the cell surface by means of specific receptors.
2. The virus is taken up in a membrane enclosed endosome by the process of receptor mediated endocytosis.
3. Uncoating takes place in the endosome and the viral RNA (genome) is released into the cytoplasm.
4. The (-)RNA of the viral genome is transported into the nucleus where it is replicated and copied by a viral enzyme into (+)RNA which is both messenger RNA and serves as a template for more (-)RNA. The (+)RNA is transported into the cytoplasm for translation into early and late viral proteins.
5. The viral core proteins are transported back into the nucleus to assemble as the capsid around the viral (-)RNA forming the "ribonucleoprotein core" or the genome-containing nucleocapsid of the virus. The viral envelope proteins assemble themselves in the cell membrane.
6. The nucleocapsid recognizes specific points on cell membrane where viral proteins have become inserted and buds off of the membrane to be released during enclosure in the viral envelope.
Replication in DNA viruses
Herpes virus – is a dsDNA virus which has a genome of 160,000 base pairs. Once the viral DNA is released into the host cell, the DNA is transcribed by the host RNA polymerase to form mRNAs which synthesise early proteins. These are mostly regulatory proteins and enzymes required for virus DNA replication. Replication occurs within the host nucleus, the host DNA synthesis gradually slows down.
Parvoviruses are one of the simplest DNA viruses. Genome = ssDNA of about 4,800 bases. The virus uses host cell enzymes for its biosynthetic processes. The viral DNA can only be replicated in the nucleus during the S phase of the cell cycle, which is when the host cell is trying to replicate its own DNA. Because the DNA is ss, the host DNA polymerase is tricked into copying it. The parvovirus genome folds back onto it-self to form a primer for replication which is then recognised by the host DNA polymerase and DNA replication continues.
Late genes are involved in the synthesis of caspid proteins, which self-assemble to form the caspid. It is seen that during the formation of the icosahedral empty procaspids are formed and then the nucleic acid is inserted. The site of morphogenesis varies between different viruses. The assembly of the caspid for enveloped and naked viruses is similar (except for poxviruses). Large clusters of paracrystaline clusters of either complete virions or procaspids are seen at the site of virus maturation. These are assembled in the cytoplasm by complex processes which begin off by the enclosure of a portion of the cytoplasmic matrix by the construction of a new membrane. The new DNAS condenses passes through the membrane and moves to the centre of the virus. Nucleoid and elliptical body construction occur within the membrane.
There are different ways in which enveloped and naked viruses are released. Naked viruses are usually released host cell lysis. Enveloped viruses have multiple steps before being released. Virus-encoded proteins are incorporated into the plasma membrane. Then the nucleocaspid is released and the envelope the envelope is formed via budding process. In many virus families, a matrix (M) protein is attached to the plasma membrane and aids in budding. Most envelopes arise from the plasma membrane. (In herpesvirus the budding and the envelope formation usually involve the nuclear membrane. (Golgi apparatus, ER, and other internal membranes may be used).
It has also been discovered that actin filaments also aid in virion release. Many viruses change the actin microfilaments in the host cell cytoskeleton.
Plan for upcoming articles in virology series
1. Influenza in more detail.
2. Herpes in more detail.
3. The relation between viruses and cancer