Viral envelope
The viral envelope (English viral envelope ) is an existing case of certain viruses external structure consisting of a lipid bilayer membrane lipids of the original host cell and viral proteins embedded in it. The viral envelope usually surrounds a capsid, in turn, the viral nucleic acid is packaged. Depending on the type of virus produced, the shell of the cell membrane on the cell surface or membrane of the endoplasmic reticulum (ER ) or Golgi apparatus in the interior of the cell.
The presence of a viral envelope is an important criterion in the classification of viruses, the so-called virus taxonomy. The enveloped viruses by the non-enveloped or " naked" viruses are deferred. While non-enveloped viruses, the infected cell must always leave by destroying the host cell, enveloped viruses without such lysis may be released. The viral envelope has a great importance in the uptake of virus into the cell, the stability against environmental influences and disinfectants as well as the ability to facilitate change in the virus surface. This variability by a viral envelope is an evolutionary advantage over non-enveloped viruses. It allows enveloped viruses, easier to subvert the immune system of a host or to better adapt to a new host. Clearly, these properties of the viral envelope, for example, the fact that all humans emerging viruses ( emerging viruses ) that pose a real or potential threat of a pandemic, enveloped viruses, such as HIV, SARS coronavirus, influenza virus, Ebola virus and West Nile virus.
- 3.1 budding at the cell membrane
- 3.2 budding at the Golgi and ER membrane
- 3.3 budding at the nuclear membrane
- 6.1 viral envelope as a virulence factor
- 6.2 virus envelope and virus inactivation
- 6.3 emergence of pandemics and "new viruses "
- 7.1 Current Literature
- 7.2 Historical Literature
Discovery
This barrier of insufficient microscopic resolution could only be overcome in the 1930s with the development of the electron microscope and by Helmut Ernst Ruska. Even the first recordings with this new technique showed outlines of viruses with oblong or round shape. However, differentiation of the fine structure of the virus and the viral envelope representation was not yet possible with the early contrast dyes. After all, beat Helmut Ruska 1943, after investigation then existing virus isolates a first classification of viruses by size and form. Until then, the viruses were classified according to the affected host and the particular disorder.
Could be that the lipid portion of the viruses in the context of a membrane structure, was then already suspected. The existence of lipid-containing double membranes in cells could already by the work of Gorter and Grendel be proved in 1925, and it was natural to assume a similar structure in lipid-containing viruses. The decisive factor was the evidence that the composition of the lipid components of the viruses that of the respective host cells resembled, in which the viruses were grown. The first indication of a viral envelope in electron micrographs can be traced retrospectively in a study by Coriell 1950. He isolated herpes simplex virus from the blisters. He observed a peculiar, round shape of the virus with a central recess, which he described as " donut- like". Today, the typical appearance of the herpes virus is referred to as " egg shape ", this means an icosahedral capsid inside surrounded by a very thick viral envelope. It was not until 1959, when a special contrasting was developed with uranium salts for electron microscopy, the structure of the virus represented a much more differentiated, so that the viral envelope could be visualized. Even today, this so-called Negative-stain is the most important method for the electron microscope pictures of viruses.
With the exploration of cellular membranes in the 1960s and 1970s was accompanied by an expansion of the understanding of the viral envelopes. This was made possible by refined techniques for structural analysis of the envelope proteins such as X-ray diffraction, freeze-fracture SEM and NMR spectroscopy, but also thanks to new considerations about the properties of biomembranes such as the fluid mosaic model of Singer and Nicholson. In the last twenty years, especially the cryo-electron microscopy provide decisive insights into the fine structure of the viral envelopes. With this technique it is possible to determine the shape and arrangement of individual coat proteins and represent a Fourier -based image processing, the viral envelope with a resolution of 0.6-1 nm.
Structure of the viral envelope
A virus envelope is always from viral coat proteins, which are embedded in a lipid bilayer. The incorporation of envelope proteins into the membrane occurs already during their synthesis on the ribosomes of the rough endoplasmic reticulum ( rER ). Either the virus envelope form already here from the membrane of the rER or the occupied areas with membrane envelope proteins are transported through the normal cellular membrane flux to the cell membrane, nuclear membrane or the Golgi apparatus. Due to the fact that the envelope proteins concentrate and agglomerate in the process of wrapping in smaller membrane surfaces, cellular membrane proteins are displaced, which are then not incorporated into the viral envelope. Due to this displacement of cellular membrane proteins, the double membrane of the viral envelope does not consist of unmodified cellular membranes, but only from their lipid content.
The proportion of the stored envelope proteins is usually so high that the lipid content on the surface at any point there is uncovered. The lipid membrane of the virus envelope is therefore no longer directly accessible to antibodies. For some viruses, such as the Hepadnaviridae the protein portion of the viral envelope is so high that the viral envelope is composed almost entirely of tightly packed envelope proteins. These are arranged very regularly and to environmental influences and detergents resistant than other enveloped viruses.
Lipid portion
The lipid membrane of the viral envelope is composed, as well as all cellular membranes, a double layer of phospholipids. They have a hydrophilic head, which forms the surface of the membrane, and two inwardly directed, lipophilic hydrocarbon chains. The phospholipids involved in the assembly of the viral envelope are phosphatidylcholine (also called lecithin), phosphatidylethanolamine, phosphatidylserine, phosphatidylinositol and sphingomyelins. The latter are present only in the outer layer of the double membrane. Still occurs, add a different high amount of cholesterol to phospholipids. The cellular membranes, and thus also the viral envelopes vary in the composition of various phospholipids and the content of cholesterol. A high cholesterol content is typical of the cell membrane, while the membranes of the endoplasmic reticulum and the Golgi apparatus are low in cholesterol. The cholesterol content of a membrane, expressed as a C / P ratio ( Molar cholesterol / phospholipid ratio ) significantly influences the morphology of a membrane so cholesterol membranes ( that is, with a typical C / P ratio of 0.4 to 0.8 ) is more stable, less flexible and with 5-6 nm by about a third thicker than low-cholesterol. Since the lipid composition of the viral envelope corresponds approximately to that of the original cell membrane, these differences are also found between viral envelopes derived from the cell membrane or intracellular membrane system.
On closer inspection, the lipid composition of most viral envelopes deviates to a lesser extent from their original membrane. How this selective uptake of lipid components occurs in the viral envelope, is still unclear. It is believed a preferred installation of different phospholipids while the aggregation of the envelope proteins in the membrane, whereby the different envelope proteins that interact strongly with the lipids and the binding of the coat proteins specific to each other phospholipids are preferred. The discovery of the so-called lipid rafts, ie in a cellular membrane floating micro-areas with high cholesterol content, has demonstrated an inhomogeneous structure of these membranes. These lipid rafts appear to be significant for the selective incorporation into the viral envelope. For some viruses, the presence of these micro-areas even for the storage of the envelope proteins and the formation of the viral envelope is a necessary condition, since they increase the density of viral envelope proteins regional and enable the aggregation. Conversely, the presence of cholesterol in the case of some viruses for virus penetration into the cell is required. Thus reduced the cultivation of canine distemper virus in cell cultures supplemented with an inhibitor of cholesterol synthesis was added, its ability to infect other cells by 80%. The same was also observed in viral envelopes of the varicella- zoster virus, which does not occur at the cell membrane cholesterol.
Table: Comparison of lipid components of typical cellular membranes ( rER: rough ER, SER: smooth ER) and viral envelopes ( human immunodeficiency virus HIV -1 antigen - spherical particles of hepatitis B virus sHBV ). For better comparability, the lipid components are each in molar percent of the lipid fraction of the liver cell of the rat from the data of MK Jain (1980 ) converted. The SER membrane also contains 2.0% diphosphatidylglycerol ( cardiolipin ).
As is apparent from a comparison of the listed in the table above lipid components, one can infer the determination of lipid composition of the viral envelope to the cellular membrane of origin. In the given example, the HIV-1 has the typical composition of the cell membrane and the spherical blank HBV particles - which correspond in the complete virion lipid composition - the lipid profile of the rough ER. The specific arrival and depletion of the components in comparison to the original membrane is also visible. Since the lipid composition of the membrane of different cell types may vary, also corresponding deviations of the viral envelope with a virus can be expected if it increased in the organism or in the cell culture in various cell types. Also, it may occur within a single cell at slightly different lipid fractions, when the cell membrane directional polarity as, for example, in cells which are arranged in a lumen. So can form different viral envelopes at the apical or basal regions of the cell membrane.
Viral envelope proteins
In a manner similar to cellular membrane proteins, the viral envelope proteins are incorporated into the lipid membrane. One or more transmembrane lipophilic protein domains crossing the lipid membrane and thus separate a smaller inner domain of a larger outer. In most envelope proteins of the carboxyl terminus is on the inside, so that the coat proteins are among the class -1 membrane proteins.
(Also called " intracellular anchor" or anchor domain ), the inward domain is hydrophilic and can mediate binding to subsequent internal structures. In the classical case this is a capsid. For viruses with multiple capsids or virus complex structure, the inner domain binds to other proteins that line the bottom of the viral envelope in addition. These lie between capsid and envelope in the matrix space and are therefore referred to as matrix proteins. In the simplest case, the inner domain of the envelope protein of a folded end of the protein. Crossing the coat protein of the lipid membrane several times ( " multipass " ), the inner domain is a resulting loop. The interaction between the inner domain, either directly or indirectly without further binding partner proteins or matrix on the capsid is the determined power to the curvature of the membrane during the wrapping.
The transmembrane domain consists of a lipophilic α -helix, the length is dictated by the thickness of the lipid membrane. Those viruses which are enveloped in the thicker cell membrane cholesterol which require 26 amino acids ( influenza virus) of the helix, for example. If the virus enveloped in the membrane of the rER, meet 18-20 amino acids ( yellow fever virus ) for a transmembrane helix. The structure elucidation of an envelope protein may therefore give an indication to which the membrane virions are formed. The viral coat protein may also have several transmembrane domains, which form helices closely spaced beams in the membrane. The envelope proteins of Flaviviridae family have two transmembrane helices, whose close ties to each other is mediated by a hydrophilic edge; these domains thus have an amphiphilic structure. Since the helices displace the lipids of the membrane, one can formulate a general rule that the lipid portion of a viral envelope is all the more smaller the more transmembrane domains have the envelope proteins.
The tasks of the outer domain - receptor binding and membrane fusion - can be combined in one envelope protein or distributed across multiple, cooperating envelope proteins. With only a few exceptions, the envelope proteins associate to give complexes of several identical or different envelope proteins. These oligomers can be visible when corresponding size in the electron microscope pictures as so-called "spikes" or Peplomere. Very characteristic spikes can be represented for example in the virus family Orthomyxoviridae and Coronaviridae; latter also received its name from this characteristic of the viral envelope.
The number of different envelope proteins, and the composition of the envelope protein oligomers characteristic of many viral species. Only one coat protein is present at the rhabdoviruses, the simple forms trimers ( [G ] 3). In retroviruses, such as Rous sarcoma virus, two glycoproteins (SU and TM) lie down together to a heterodimer, which in turn assembles with two other heterodimers to a hexamer ( [SU -TM ] 3). Alphaviruses have two (E1, E2 ) or three envelope proteins (E1 -3) that arrange after assembly into larger three complexes ( [ E1 -E2 -E3 ] 3).
Envelope proteins perform in the cell membrane and occasionally other functions during virus replication than the envelope of the virion. For this purpose, they can alternatively arrange to new structures and form, such as the SARS coronavirus pores leading to lysis of the cell.
Symmetrical viral envelopes
The inner portion of the envelope proteins can interact with an enveloped capsid such that always only a coat protein ( or a composite mounted dimer or trimer of the coat proteins ) binds to only one capsomere. Through this arrangement, the fixed shape and symmetry of the icosahedral capsid interior is transferred to the outer viral envelope and there are, despite the mobility of the lipid membrane icosahedral virus capsids strictly structured. This form of so-called " inside out morphogenesis " is found in the alphavirus genus of the Togaviridae family (e.g., Semliki Forest Virus and Sindbis virus ) and the genus Flavivirus of the family Flaviviridae.
Special shapes
In a few virus families a lipid bilayer membrane is not present as an outer enveloping structure but is located inside the virions. Especially extraordinary are here two families of bacteriophages Corticoviridae and Tectiviridae in which the lipid membrane is located inside an icosahedral capsid. This structure is not known as the viral envelope, as it is neither outside nor fulfilled typical tasks of a viral envelope as the attachment to the cell surface. Existing in the Tectiviridae membrane vesicle is by attachment of the capsid of the bacteria to the active surface penetration of the double -stranded bacteriophage DNA into the host cell.
When representatives of the Poxviridae family, the viral envelope consists of an outer and inner double membrane addition. Within the cytosol, the poxviruses are a simple wrapper. This first enclosure is not created by budding from a cellular membrane, but by assembling a completely new lipid membrane on the outside of the still immature, later double concave capsid. For reconstruction of the membrane degraded membrane components from the transition region between the Golgi and ER membrane ( Intermediate compartment ) were used. The simple enveloped virus particles is then obtained by budding at the Golgi membrane a second, outer viral envelope.
Formation during virus replication
The synthesis of the envelope proteins and the formation of the viral envelope marked the final stage in the reproductive cycle of an enveloped virus; before the viral genome must be replicated and eventually packaged in a capsid.
The process of wrapping a virus, even budding ( "budding " ) called, corresponds to a specific package in a constricted membrane vesicles. Inside the cell, the constant formation and fusion of membrane vesicles is a physiological process of mass transport, the so-called exocytosis and endocytosis. An enveloped virus uses these existing properties and mechanisms of membrane flow in which it modifies this and controlled by the viral structural proteins. The energy that is necessary for the curvature of the lipid membrane and the formation of bubbles, comes only the interaction of the envelope proteins with each other, the envelope proteins with internal structures, such as matrix proteins and capsids or capsid with the lipid membrane; a supply of energy, for example in the form of ATP is not necessary for this purpose. The energetically more favorable aggregation of the envelope proteins to overcome, for example, in the Togaviruses for the lipid membrane energetically favorable curvature by hydrogen bonds, ionic bonds, and in particular by hydrophobic interactions. The formation of the viral envelope - no matter on which membrane system - is therefore only controlled by the translation, the transport and concentration of viral proteins on the respective membrane compartment. Budding as a spontaneous assembly of capsid, lipid membrane and envelope proteins in view of their thermodynamic consideration of great interest. To their descriptive models have been widely used in order to calculate the interaction of the components involved can. In accordance with the measured in-vitro duration of budding takes to complete, could also be calculated in the model 10 to 20 minutes. As limiting the diffusion processes of the envelope proteins along the lipid membrane and the displacement of water molecules between the cover and capsid are derived. The model calculations suggest a preferential budding of viruses in those places of cellular membranes expect that already have morphologically present a curvature of the lipid membrane. This agrees with the electron microscopy of infected cells, where budding viruses were predominantly found on the curved sides of the Golgi apparatus or the cell membrane.
The mechanisms of the enclosure are similar to the respective cellular membranes in principle. The first model was developed for budding by the example of the Semliki Forest virus (SFV ). Here the binding of the intracellular anchor of the envelope proteins leads to an already closed capsid to the curvature of the lipid membrane. From this, the thesis was concluded that the emergence of a viral envelope is imperative for the presence of envelope proteins and binding to capsid proteins. This early model has been greatly restricted, as it retroviruses an envelope of capsids without the presence of the envelope proteins ( Env protein ) observed when the capsid proteins (Gag proteins) are alone in transfected cell cultures available.
In addition to the special case of the Gag protein of retroviruses, there are three other important variants of the budding ( see illustration). The simplest way is the slight curvature of the lipid membrane by the interaction of the envelope proteins on their inner anchoring domains. A closed capsid binds to these envelope proteins aggregate and drives through the interaction with the envelope proteins budding ahead ( see figure, case A). In the second case (B), the assembly of the capsid occurs only upon binding to the envelope proteins. The interaction of capsid proteins and envelope proteins allows only the binding of the nucleic acid and completes the virus during budding. This variant can also be supplemented by a mediating between envelope and capsid protein matrix. With viruses that do not have symmetric capsids (e.g., the Bovine Viral Diarrhea Virus, and the related hepatitis C virus ), the binding of the nucleic acid to basic proteins ( nucleotide or core proteins ) similar to matrix proteins at the inner side of the membrane with sufficient envelope proteins interact.
- Formation of the viral envelope by budding: TEM images of infected cell cultures
Lassa virus in the late stage of budding of the viral envelope
Human herpesvirus 6 after release at the cell membrane
Enveloped particles of Rift Valley fever virus ( Bunyaviridae ) in the lumen of the ER
Budding at the cell membrane
The formation of the viral envelope to the cell membrane requires first transport of the envelope proteins on the cell surface. The viral proteins are produced at the ribosomes of the rough ER, the envelope proteins even during synthesis, with its transmembrane domain pierce the membrane of the ER and can be stored in them. Via the system membrane of the Golgi apparatus, the envelope proteins are glycosylated. The now modified ( mature ) envelope proteins are transported into constricted, exocytotic vesicles to the cell membrane and fuse with it. Those domains of the envelope proteins, which were previously directed into the lumen of the ER, now located extracellularly. The routed to the cell membrane remaining virus components ( capsid, nucleic acid and any matrix proteins), can now be wrapped. The formation mechanism of the cell membrane requires the incorporation of viral envelope proteins, which can lead to the already mentioned formation of syncytia (see section envelope proteins ). However, this presented outwardly viral proteins can be detected in addition as foreign by immune cells, such that an early immune response against the envelope proteins can be carried out. All viruses whose shells are derived from the cell membrane, are also taken up by fusion of the envelope with the cell membrane. This type of recording ( "fusion from without " ) allows an infection without a transport in an endosome.
Budding at the Golgi and ER membrane
Budding at the nuclear membrane
The members of the virus family Herpesviridae are in their construction, their reproductive strategy and also in the formation of the viral envelope is a special case, since the very large capsids of herpesviruses are assembled in the nucleus, in which the double-stranded DNA of the virus is synthesized. Already in very early electron microscopic investigations of cells in which multiplies the herpes simplex virus, could be seen budding capsids at the nuclear membrane and inside of the enveloped virus particles in the surrounding the core perinuclear cistern. Since the perinuclear cistern is connected by membrane tubes with the rough ER, it was assumed that the enveloped virions are then funneled through membrane vesicles of the Golgi apparatus of the cell. A study of the lipid composition of the viral envelope, however, revealed that the lipid components do not correspond to those of the nuclear membrane, but also possess the lipid profile of the Golgi membranes. This finding led to the discovery that the herpesviruses gain first by budding at the nuclear membrane, a viral envelope. However, these fused again with the outer membrane of the perinuclear cistern and is so naked capsid in the cytosol -free. Only through a second budding into a vesicle membrane abgeschnürtes of the Golgi apparatus, which is enriched with viral envelope and matrix proteins, the capsid acquires its final envelope. These so-called secondary Behüllung then corresponds to only the viral envelope of the released viruses.
Empty viral envelopes and " Defective viruses "
In some viruses, the envelope proteins are capable of, without a further connection to an internal structure produce a budding. This is particularly the case when the interaction between the inner anchor domains of the envelope proteins is particularly high. The result is empty or incompletely filled viral envelopes. The existence of these empty shells was first " Australia antigen " discovered in studies of the so-called, led by B. Blumberg for the discovery of the hepatitis B virus ( HBV). The antigen discovered consists of the three envelope proteins of HBV ( HBsAg ). In the blood of HBV -infected patients, the HBs antigen predominantly in empty spherical particles with a diameter of 22-24 nm and empty tube-like structures ( " tubules " ) is to be found of variable length. Below about 1,000 to 10,000 HBs antigen - containing particles only is an infectious, complete virus ( 42 nm) to prove. This huge surplus of empty viral envelopes primarily serves to neutralize antibodies against the envelope protein and thus prevent their binding to the complete virus.
Empty viral envelopes, which are often smaller as in the example of the complete HBV viruses, even with a faulty or incomplete packaging of segmented genomes (eg, influenza virus) found when they are grown in cell cultures. These particles are called " defective interfering particles" (DIP ), or " virus-like particles " (VLP ) refers to. The hepatitis C virus, the existence of incomplete particles in the blood serum of patients has been suggested as an alternating stoichiometric ratio of the core protein is detectable at the RNA.
A particular example of the viral envelope provides the hepatitis D virus, because it itself has no genes for coat proteins with proper packaging. It is dependent on the presence of HBV in the same cell, because it can be packaged with the envelope proteins of HBV and released. It is therefore referred to as a defective or dependent virus ( Virusoid ).
The capsids of enveloped and non- enveloped viruses
In viral envelopes with high lipid content, the envelope proteins are arranged flexible and can move in the membrane sideways. This liquid characteristic of the virus envelope means that a closed envelope is present even if an error in the arrangement of the envelope proteins, or a gap in the surface of symmetry occurs. Such a misalignment would at naked viruses lead to a lack of protection of the genome or to the disintegration of the capsid. Under the protection of a viral envelope consists of the structure of the capsid compared to non-enveloped viruses greater freedom because they are no longer directly related to the protection of the genome from nucleases or represent a target for the immune system. The capsids of enveloped viruses may therefore also have gaps or only net-like clothe the genome. This has in retroviruses and the closely related hepadnaviruses (eg hepatitis B virus) is of great importance, since the still non-enveloped, but closed capsid can still take up ATP and nucleotides during propagation to complete the already packaged genome. The capsids of some viruses enveloped the gaps also allow for a release of the genome, for example, to the nuclear pore without the capsid in the cytosol must disintegrate before.
In addition to pure imitation economic endogenous proteins for camouflage, the envelope proteins can also mimic binding properties of the host proteins. Wherein the lentivirus genus of the retrovirus, the similarity of the outer domain of the envelope protein gp41 has been described with interleukin -2; here the binding of interleukin receptors is imitated by immune cells, which are regarded as target cells of these viruses.
The ability of retroviruses in cell culture without a separate coat protein to induce a budding is used in the genetic engineering production of artificial viral particles to produce particles having modified surface properties. So foreign envelope proteins can be incorporated into the shell of this so-called pseudo- types, for example, the binding of these receptors to be able to examine or use them in research as viral vectors. The formation of pseudo- types seem to be linked to the existence of the previously mentioned lipid rafts.
Also in natural infections, the formation of pseudotypes described. Thus, two types of virus with simultaneous infection of a cell, the different envelope proteins mixed store in an emerging coating or a virus can be completely packed with the shell of another virus. This phenomenon of pseudo - type formation is also referred to as a phenotypic mixture ( " Phenotypic mixing" ).
Prevent the loss of the viral envelope or the removal of the lipid components of the shell, that the enveloped virus is capable of infecting the host cell. It is being used for the inactivation of enveloped viruses to prevent the spread of the virus. The most sensitive component of the viral envelope, the lipid membrane may be damaged by grease-dissolving alcohols, such as ethanol or 2 -propanol. At a high lipid content of the viral envelope as in the orthomyxoviruses suffice mild detergents or soaps to reduce the infectivity of the virus. With the inactivation of possible enveloped viruses such as HIV, HBV and HCV in blood product transfusion a combination of mild detergents and solvents can be used.
Emergence of pandemics and "new viruses "
The high immunological flexibility of the envelope proteins allows some enveloped viruses to multiply in different host species. Thus, infections can occur across species new or intermediate hosts are used as intermediaries. The arthropods (eg, mosquitoes and ticks ) transmitted viruses, called arboviruses, therefore, are predominantly enveloped viruses. The only non-enveloped Coltivirus genus, whose members may be transferred as arboviruses have to replace the flexibility of the viral envelope, a second capsid. Viruses are usually then particularly pathogenic when they occur in a new host population. Therefore, the enveloped viruses that favor the host transition from animal to man especially a particularly high potential for emerging infections in humans.