A viral envelope is the outermost layer of many types of
virus
A virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism. Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and archaea.
Since Dmitri Ivanovsk ...
es.
It protects the genetic material in their life cycle when traveling between host cells. Not all viruses have envelopes.
Numerous human pathogenic viruses in circulation are encased in lipid bilayers, and they infect their target cells by causing the viral envelope and cell membrane to fuse. Although there are effective vaccines against some of these viruses, there is no preventative or curative medicine for the majority of them. In most cases, the known vaccines operate by inducing antibodies that prevent the pathogen from entering cells. This happens in the case of enveloped viruses when the antibodies bind to the viral envelope proteins. The membrane fusion event that triggers viral entrance is caused by the viral fusion protein. Many enveloped viruses only have one protein visible on the surface of the particle, which is required for both mediating adhesion to the cell surface and for the subsequent membrane fusion process. To create potentially protective vaccines for human pathogenic enveloped viruses for which there is currently no vaccine, it is essential to comprehend how antibodies interact with viral envelope proteins, particularly with the fusion protein, and how antibodies neutralize viruses. Enveloped viruses enter cells by joining a cellular membrane to their lipid bilayer membrane. Priming by proteolytic processing, either of the fusion protein or of a companion protein, is necessary for the majority of viral fusion proteins. The priming stage then gets the fusion protein ready for triggering by the processes that go along with attachment and uptake, which frequently happens during transport of the fusion protein to the cell surface but may also happen extracellularly. So far, structural studies have revealed two kinds of viral fusion proteins. These proteins are believed to catalyze the same mechanism in both situations, resulting in the fusing of two bilayers. In other words, these proteins operate as enzymes, which while having various structural variations catalyze the same chemical reaction.
The envelopes are typically derived from portions of the host
cell membrane
The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of all cells from the outside environment ( ...
s (
phospholipids
Phospholipids, are a class of lipids whose molecule has a hydrophilic "head" containing a phosphate group and two hydrophobic "tails" derived from fatty acids, joined by an alcohol residue (usually a glycerol molecule). Marine phospholipids t ...
and proteins), but include some viral
glycoproteins. One of the main parts of human pathogenic viruses is glycoprotein. They have been shown to play significant roles in immunity and infection. Viral glycoproteins, a new class of cellular inhibitory proteins has been discovered. These include the E3 ubiquitin ligases of the membrane-associated RING-CH (MARCH) family, which among other things, inhibits the expression of cell surface proteins implicated in adaptive immunity. Being made up mostly of host membrane, the viral envelope can also have the proteins associated with the host cell within their membrane after budding. Many enveloped viruses mature by budding at the plasma membrane, which allows them to be discharged from infected cells. During this procedure, viral transmembrane proteins, also known as "spike" proteins, are integrated into membrane vesicles containing viral core components. For a very long time, it was thought that the spike proteins, which are necessary for infectivity, were directly incorporated into the viral core through their cytoplasmic domains. Recent research suggests that while such direct interactions may be what causes the budding of alphaviruses, this may not be the case for retroviruses and negative strand RNA viruses. These viruses can form bud particles even in the absence of spike proteins by relying only on viral core components. The spike proteins can occasionally be produced as virus-like particles without the viral core. Therefore, optimal budding and release may be dependent on a coordinated "push-and-pull" action between core and spike, where oligomerization of both components is essential. They may help viruses avoid the
host immune system
The immune system is a network of biological processes that protects an organism from diseases. It detects and responds to a wide variety of pathogens, from viruses to parasitic worms, as well as cancer cells and objects such as wood splint ...
. TAM receptor tyrosine kinases increase phagocytic clearance of apoptotic cells and inhibit immunological responses brought on by Toll-like receptors and type I interferons when they are activated by the ligands Gas6 and Protein S. (IFNs). The phospholipid phosphatidylserine may be seen on the membranes of several enveloped viruses, which they employ to bind Gas6 and Protein S to activate TAM receptors. Ligand-coated viruses stimulate type I IFN signaling, activate TAM receptors on dendritic cells (DCs), and suppress type II IFN signaling to circumvent host defenses and advance infection. TAM-deficient DCs exhibit type I IFN responses that are more pronounced than those of wild-type cells in response to viral exposure. As a result, flaviviruses and pseudo typed retroviruses have a harder time infecting TAM-deficient DCs, albeit infection can be brought back by type I IFN antibodies. A TAM kinase inhibitor, meanwhile, prevents infection of wild-type DCs. TAM receptors, which are potential targets for therapy, are thereby activated by viruses to reduce type I IFN signaling. Glycoproteins on the surface of the envelope serve to identify and bind to
receptor sites on the host's membrane. The particular set of viral proteins are engaged in a series of structural changes. When these changes are set/finished, there is then and only then, fusion with the host membrane. These glycoproteins mediate the interaction between virion and host cell, typically initiating the fusion between the viral envelope and the host's cellular membrane. In some cases, the virus with an envelope will form an endosome within the host cell. There are three main types of viral glycoproteins: Envelope proteins, membrane proteins, and spike proteins (E, M, and S). The viral envelope then fuses with the host's membrane, allowing the capsid and viral
genome
In the fields of molecular biology and genetics, a genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding g ...
to enter and infect the host.
All enveloped viruses also have a
capsid
A capsid is the protein shell of a virus, enclosing its genetic material. It consists of several oligomeric (repeating) structural subunits made of protein called protomers. The observable 3-dimensional morphological subunits, which may or ma ...
, another protein layer, between the envelope and the genome.
The virus wraps its delicate nucleic acid with a protein shell known as the capsid, from the Latin capsa, meaning "box," in order to shield it from this hostile environment. Similar to how numerous bricks come together to form a wall, the capsid is made up of one or more distinct protein types that repeatedly repeat to form the whole capsid. This repetitive pattern creates a robust but rather flexible capsid. The nucleic acid inside the capsid is appropriately protected by its modest size and physical difficulty in opening it. The nucleocapsid of the virion is made up of the nucleic acid and the capsid. Remember that the genomes of most viruses are very small. Genes code for instructions to make proteins, so small genomes cannot code for many proteins. Therefore, the virion capsid consists of one or only a few proteins that repeat over and over to form the structure. The viral nucleic acid would be physically too large to fit inside the capsid if it consisted of more than a few proteins.The capsid, having a focused role of protecting the genome in addition to immune recognition evasion. The viral capsid is known for its protection of RNA before it is inserted into the host cell, unlike the viral envelope which protects the protein capsid.
The cell from which a virus
buds often dies or is weakened, and sheds more viral particles for an extended period. The
lipid bilayer
The lipid bilayer (or phospholipid bilayer) is a thin polar membrane made of two layers of lipid molecules. These membranes are flat sheets that form a continuous barrier around all cells. The cell membranes of almost all organisms and many vir ...
envelope of these viruses is relatively sensitive to
desiccation, heat, and
amphiphiles such as
soap
Soap is a salt of a fatty acid used in a variety of cleansing and lubricating products. In a domestic setting, soaps are surfactants usually used for washing, bathing, and other types of housekeeping. In industrial settings, soaps are use ...
and
detergents, therefore these viruses are easier to sterilize than non-enveloped viruses, have limited survival outside host environments, and typically must transfer directly from host to host. Viral envelope persistence, wither it be enveloped or naked, are a factor in determining longevity of a virus on inanimate surfaces. Enveloped viruses possess great adaptability and can change in a short time in order to evade the immune system. Enveloped viruses can cause
persistent infections.
Vaccination against enveloped viruses can function by neutralizing the glycoprotein activity with antibodies.
Examples of enveloped viruses
The following are some examples of enveloped viruses:
* DNA viruses
**
Herpesviruses
**
Poxviruses
''Poxviridae'' is a family of double-stranded DNA viruses. Vertebrates and arthropods serve as natural hosts. There are currently 83 species in this family, divided among 22 genera, which are divided into two subfamilies. Diseases associated wit ...
**
Hepadnaviruses
**
Asfarviridae
* RNA viruses
**
Flavivirus
''Flavivirus'' is a genus of positive-strand RNA viruses in the family ''Flaviviridae''. The genus includes the West Nile virus, dengue virus, tick-borne encephalitis virus, yellow fever virus, Zika virus and several other viruses which may cau ...
es
**
Alphavirus
''Alphavirus'' is a genus of RNA viruses, the sole genus in the ''Togaviridae'' family. Alphaviruses belong to group IV of the Baltimore classification of viruses, with a positive-sense, single-stranded RNA genome. There are 32 alphaviruses ...
es
**
Togavirus
''Alphavirus'' is a genus of RNA viruses, the sole genus in the ''Togaviridae'' family. Alphaviruses belong to group IV of the Baltimore classification of viruses, with a positive-sense, single-stranded RNA genome. There are 32 alphaviruses, ...
es
**
Coronaviruses
**
Hepatitis D
Hepatitis D is a type of viral hepatitis caused by the hepatitis delta virus (HDV). HDV is one of five known hepatitis viruses: A, B, C, D, and E. HDV is considered to be a satellite (a type of subviral agent) because it can propagate only in ...
**
Orthomyxovirus
''Orthomyxoviridae'' (from Greek ὀρθός, ''orthós'' 'straight' + μύξα, ''mýxa'' 'mucus') is a family of negative-sense RNA viruses. It includes seven genera: '' Alphainfluenzavirus'', '' Betainfluenzavirus'', '' Gammainfluenzavirus'' ...
es
**
Paramyxoviruses
**
Rhabdovirus
''Rhabdoviridae'' is a family of negative-strand RNA viruses in the order '' Mononegavirales''. Vertebrates (including mammals and humans), invertebrates, plants, fungi and protozoans serve as natural hosts. Diseases associated with member vir ...
**
Bunyavirus
''Bunyavirales'' is an order of segmented negative-strand RNA viruses with mainly tripartite genomes. Member viruses infect arthropods, plants, protozoans, and vertebrates. It is the only order in the class ''Ellioviricetes''. The name ''Bunyavi ...
es
**
Filovirus
''Filoviridae'' () is a family of single-stranded negative-sense RNA viruses in the order ''Mononegavirales''. Two members of the family that are commonly known are Ebola virus and Marburg virus. Both viruses, and some of their lesser known re ...
es
* Retroviruses
**
Retroviruses
Examples of non-enveloped viruses
The following are some examples of viruses without envelopes:
* DNA viruses
**
Adenoviruses
Adenoviruses (members of the family ''Adenoviridae'') are medium-sized (90–100 nm), nonenveloped (without an outer lipid bilayer) viruses with an icosahedral nucleocapsid containing a double-stranded DNA genome. Their name derives from the ...
**
Parvoviruses
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Polyomaviruses
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Anelloviruses
**
* RNA viruses
**
Caliciviruses
**
Picornavirus
Picornaviruses are a group of related nonenveloped RNA viruses which infect vertebrates including fish, mammals, and birds. They are viruses that represent a large family of small, positive-sense, single-stranded RNA viruses with a 30 nm ...
es
**
Reoviruses
**
Astrovirus
Astroviruses are a type of virus that was first discovered in 1975 using electron microscopes following an outbreak of diarrhea in humans. In addition to humans, astroviruses have now been isolated from numerous mammalian animal species (and a ...
es
**
Hepeviridae
''Hepeviridae'' is a family of viruses. Human, pig, wild boar, sheep, cow, camel, monkey, some rodents, bats and chickens serve as natural hosts. There are two genera in the family. Diseases associated with this family include: hepatitis; high ...
**
See also
*
Bacterial capsule
The bacteria capsule is a large structure common to many bacteria. It is a polysaccharide layer that lies outside the cell envelope, and is thus deemed part of the outer envelope of a bacterial cell. It is a well-organized layer, not easily was ...
*
Cell envelope
The cell envelope comprises the inner cell membrane and the cell wall of a bacterium. In gram-negative bacteria an outer membrane is also included. This envelope is not present in the Mollicutes where the cell wall is absent.
Bacterial cell env ...
References
External links
*
{{Viral proteins
Envelope
An envelope is a common packaging item, usually made of thin, flat material. It is designed to contain a flat object, such as a letter or card.
Traditional envelopes are made from sheets of paper cut to one of three shapes: a rhombus, a sh ...