Course: Virology
Lecturer: Dr. Weam SaadLecture: Viral Envelopes and proteins
3. Viral Envelopes
The viral envelope, characteristic of virus families, is derived from membranes of the host cell by budding, which occurs during the release of the virions from the cell. This membrane is mainly a piece of the plasma membrane; or it may be part of the Golgi Apparatus, endoplasmic reticulum or the nuclear membrane, depending upon the virus and where the replication takes place inside the cell.Generally, the envelope is composed by a lipid bilayer (of cellular origin) and associated proteins. The proteins associated with the lipid bilayer are from the viral origin (virus encoded genes) and they are mainly glycoproteins. The number of viral proteins in the envelope may contain from one type to more than ten, depending on the virus.
Virus envelope glycoproteins do several functions, including the initial attachment of the virion to the target cell, penetration, fusion, and cell-to-cell spread. The attachment of a virion to the cellular surface requires intact envelope. Antiviral drugs that are used against the envelope proteins can decrease the ability of the virus to attach and initiate infection, then decreasing infectivity.
The process of budding for getting the envelope by the newly formed virions, can cause the either the death of the host cell or the infected cell can stay a life. If many virions are released, the integrity of the host cell membrane may be affected and lead to death of the cell. But, when the release of virions is slow can lead to chronic shedding and persistent infections. The non-enveloped viruses released from the cell through cell lysis and cause host cell death, the enveloped viruses cause cell survival during budding process without leading to cell death after viruses release from infected cells.
4. Viral Proteins
The number of proteins encoded by viral genomes differs from two proteins to over hundreds. There are two basic types of virus-encoded proteins:
Structural proteins:
They are part of the physical structure of the virion (capsid, envelope). They form the capsid and package the nucleic acid genome. In some enveloped viruses, there is a protein layer between the capsid and the envelope (the tegument).
External structural proteins of the capsid or envelope are ligands, which interact with receptors on the surface of target cells. Some of these proteins (glycoproteins) are processed in the lumen of the rough endoplasmic reticulum, where oligosaccharides are attached to the polypeptide chain. They are then sent to the Golgi apparatus, to secretory vesicles, and then fuse with the plasma membrane where they are present on the surface of the infected cell. This is especially important for enveloped viruses.
The structural proteins of viruses have several important functions:
Increase the transfer of the viral nucleic acid from one host cell to another.
Protect the viral genome against inactivation by nucleases.
Participate in the attachment of the virus particle to a susceptible cell.
Form the structural symmetry of the virus particle.
Determine the antigenic characteristics of the virus. The host’s protective immune response is directed against antigenic determinants of proteins or glycoproteins exposed on the surface of the virus particle. Some surface proteins may also have specific activities (eg, influenza virus hemagglutinin agglutinates red blood cells).
Envelope glycoproteins play roles in mediating interactions between the virions and host cells (attachment, penetration, fusion, cell-to-cell spread) and are major targets for neutralizing antibodies produced by host.
Non-structural proteins:
They are produced inside infected cells and play roles in different steps of viral replication. Some viruses encode several nonstructural proteins that play roles in the regulation of viral and cellular gene expression, regulation of different steps of the viral cycle, against host defenses, cell transformation.
Some viruses carry enzymes (which are proteins) inside the virions. The enzymes are present in very small amounts and they are essential for the initiation of the viral replicative cycle when the virion enters a host cell. Examples of these proteins include:
RNA polymerase carried by viruses with negative-sense RNA genomes (eg, orthomyxoviruses, rhabdoviruses) that are needed to copy the first mRNAs.
Reverse transcriptase is an enzyme in retroviruses that makes a DNA copy of the viral RNA, which makes a DNA copy of a RNA template. This step is an important character of retroviruses whose RNA needs to be converted to DNA in order to be incorporated into the host chromosome . E.g. poxviruses, the cores of which contain a transcriptional system; many different enzymes are packaged in poxvirus particles.
(RNA polymerase)
(Reverse transcriptase)
5. Other Viral Components
Lipids:The lipids of viruses are derived from the cellular membranes of the host cell. These are composed mainly of phospholipids (50 - 60%) and cholesterol, these include the lipid bilayer of the host membranes surrounding the virion of enveloped viruses and represents approximately 20 - 35% dry weight.
Carbohydrates:
The carbohydrates of viruses are oligosaccharide (side chains of glycoproteins, glycolipids, and mucopolysaccharides). The composition of the carbohydrates depends of the host cell. Viral carbohydrates are mainly found in the envelope.