Biological Sciences
Viral Envelopes
Viral envelopes are outer layers of some viruses, composed of a lipid bilayer derived from the host cell membrane. These envelopes play a crucial role in the virus's ability to infect host cells and evade the immune system. They contain viral glycoproteins that facilitate attachment to host cells and fusion of the viral envelope with the host cell membrane during the infection process.
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8 Key excerpts on "Viral Envelopes"
eBook - PDFMammalian Cell Membranes
Responses of Plasma Membranes
- G. A. Jamieson, D. M. Robinson(Authors)
- 2014(Publication Date)
- Butterworth-Heinemann(Publisher)
Such a model would agree with the trilaminar hypothesis of Danielli and Davson (1935) and implies that it is primarily the polar groups of the lipids which HOST CELL MEMBRANES IN ANIMAL VIRUS REPRODUCTION 275 interact with the envelope proteins. These proteins have a strong affinity for lipids as suggested by experiments of Cohen, Atkinson and Summers (1971). Moreover, they seem to require a specific lipid pattern with which to interact. This they find in certain cellular membranes such as the plasma membrane. It cannot be ruled out that the envelope proteins might exert some limited selectivity in utilizing such lipids. On the other hand, a certain flexibility in the interaction of the proteins and the lipids seems to be possible. Therefore, variations in the lipid composition of the membranes of different host cells are reflected in the virions. 9.3 THE ROLE OF MEMBRANES IN THE BIOSYNTHESIS OF NONENVELOPED VIRUSES Nonenveloped viruses are not surrounded by a membrane-like structure and they do not contain constituents derived from cellular membranes. Never-theless, the replication of these viruses is closely associated with cellular membranes, too. This association has been investigated in detail in the picornavirus group. Electron-microscopic studies have shown that in picornavirus-infected cells an extensive proliferation of smooth membranes occurs, forming channels and cisternae enclosing islets of cytoplasm in the centrosphere region of infected cells (Dales et al, 1965; Amako and Dales, 1967). The mem-branous structures first become recognizable 3 hours after infection of cells with polio virus, and increase steadily. Seven hours after infection, they com-pletely fill the central portion of the cell, displacing the nucleus to one side. Mosser et al (1972) carried out cell fractionation studies of HeLa cells in-fected with polio virus.
eBook - PDF- Edward K. Wagner, Martinez J. Hewlett, David C. Bloom, David Camerini(Authors)
- 2009(Publication Date)
- Wiley-Blackwell(Publisher)
Few (if any) viral genes directed toward lipid biosynthesis or membrane assembly are yet identified. While the lipid bilayer is entirely cellular, the envelope is made virus specific by the insertion of one or several virus-encoded membrane proteins that are synthesized during the replication cycle. Some of the patterns of envelopment at the plasma membrane for viruses that assemble in the cytoplasm are shown in Fig. 6.8. Viral glycoproteins, originally synthesized at the (a) (b) (e) (d) (c) Nucleus Viral glycoproteins transported to plasma membrane in vesicle Vesicle containing viral glycoproteins Host glycoproteins in plasma membrane Viral envelope glycoproteins Glycosylation starts in rough endoplasmic reticulum Glycosylation continues in golgi apparatus Budding virion Free infectious virus Bud Nucleocapsid forms Migrates to virus-modified membrane Synthesis and co-translational membrane insertion of viral glycoproteins Fig. 6.8 Insertion of glycoproteins into the cell’s membrane structures and formation of the viral envelope. The formation of viral glycoproteins on the rough endoplasmic reticulum parallels that of cellular glycoproteins except that viral mRNA is translated (a). Full glycosylation takes place in the Golgi bodies, and viral glycoproteins are incorporated into transport vesicles for movement to the cell membrane where they are inserted (b). At the same time (c), viral capsids assemble and then associate with virus-modified membranes. This can involve the interaction with virus-encoded matrix proteins that serve as “adapters.” Budding takes place (d,e) as a function of the interaction between viral capsid and matrix proteins and the modified cellular envelope containing viral glycoproteins.
eBook - ePub- Volkmar Weissig, Gerard G. D'Souza(Authors)
- 2011(Publication Date)
- Wiley(Publisher)
The scale bar is 100 µm for each image. The capsid is indicated by the asterisk and the lipid bilayer (best seen in the cryoelectron microscopy image) is indicated by the arrowhead. In each case the eGPs project from the lipid bilayer but are poorly visible. Biochemical composition is a third criterion used to classify viruses and is the most relevant to later sections of this chapter. All viruses contain protein, either encoded by the virus genome or taken from infected cells, but a major distinction is the presence of a bounding lipid bilayer that is also derived from the cell membrane when virus buds from the cell. Viruses that have lipid membranes are termed enveloped viruses and will be the main subject of this chapter. They are special, as embedded in the membrane are the viral eGPs. Often called “spike” proteins, the eGPs project from the virus membrane and are responsible for the initial interaction with the cell as well as catalyzing fusion of the virus and cell membrane. Since the lipid bilayer encapsulates the virus core, fusion of virus and cell membranes results in the seamless merging of virus and cell membranes to release the core into the cell cytoplasm. The ability of the eGP to induce membrane fusion of lipid bilayers, and the fact that the majority of eGPs are structurally distinct functional units that operate independently from the other virus proteins that underlie the membrane, offers great advantages over nonenveloped virus proteins since the size and composition of the cargo are not restricted and simply need to be bound in a lipid bilayer. 20.3 ENVELOPE GLYCOPROTEIN STRUCTURE, FUNCTION, AND CLASSIFICATION eGPs are multifunctional proteins that bind to specific surface-exposed receptors on cells and allow the virus to penetrate into the cell cytoplasm. Both these properties are highly desirable aspects of a nanocargo delivery mechanism
eBook - PDFThe Glycoconjugates V4
Glycoproteins, Glycolipids and Proteoglycans
- Martin Horowitz(Author)
- 2012(Publication Date)
- Academic Press(Publisher)
II. STRUCTURE OF VIRUS MEMBRANES The membranes of enveloped viruses consist of an outer layer of glycopro-teins, which in the electron microscope is seen as projections or spikes. The spikes can be removed by digestion with proteolytic enzymes, such as bromelain (Brand and Skehel, 1972) or thermolysin (Utermann and Simons, 1974; Mudd, 1974). After removal of the hydrophilic glycoprotein moiety, lipophilic peptides remain associated with the lipids, apparently anchoring the polypeptide to the lipid bilayer (Utermann and Simons, 1974; Skehel and Waterfield, 1975; Schloemer and Wagner, 1975c; Chads and Morrison, 1979). The lipids of Sindbis virus and Semliki Forest virus (SFV)* envelope are arranged in a bilayer form, as shown by small-angle X-ray scattering analysis. The polar groups of phospholipids are at a radial distance of 202 and 258 A for Sindbis virus (Harrison et al., 1971) and 202 and 250 A for SFV (S. C. Harrison and L. Kaariainen, unpublished; see also Kaariainen and Soderlund, 1978). The lipids of SFV consist of 16,000-17,000 phospholipid and cholesterol molecules per virion, together with a small amount of glycolipids and neutral lipids (Renkonen et al., 1971; Laine et al., 1973). The lipids of other enveloped viruses are probably also in a bilayer form and consist mainly of phospholipids * Abbreviations: Viruses —VSV, vesicular stomatitis virus; SFV, Semliki Forest virus; FPV, fowl plague virus; SV5, simian virus 5; NDV, Newcastle disease virus; SSH, snowshoe hare virus; TVT, Trivittatus virus; RSV, Rous sarcoma virus; ASV, avian sarcoma virus; MuLV, murine leukemia virus; MuMTV, murine mammary tumor virus; LCM, lymphocytic chorion meningitis virus; MHV, murine hepatitis virus; HSV, Herpes simplex virus; EEV, extracellular enveloped virus (vaccinia). Monosaccharides—NeuAc, sialic acid; Gal, galactose; GlcNAc, yV-acetylglucosamine; Man, man-nose; Fuc, Fucose. 3 Virus Glycoproteins and Glycolipids 195 and cholesterol.
eBook - PDF- Nigel J. Dimmock, Andrew J. Easton, Keith N. Leppard(Authors)
- 2015(Publication Date)
- Wiley-Blackwell(Publisher)
Finally, virus infection can itself reduce the levels of specific receptors on the cell surface. Binding to receptors will typically cause their internalization and this often results in the receptors being trafficked to lysosomes for degradation. Also, many enveloped viruses express their glycoproteins, which include the attachment functions, on the surface of infected cells prior to their inclusion into envelopes of progeny particles. By binding here with virus receptor molecules, they can again interfere with receptor trafficking to reduce receptor levels on the cell surface. A practical consequence of this is that an infected cell is often refractory to infection, at least for a time, by any other virus that uses the same receptor. An extreme example of this is retroviruses, which typically do not cause acute cytolysis upon infection but which establish themselves in the DNA of the cells (see Chapter 9). Retroviral sequences can become incorporated into the germ line of the organism and, thereafter, the proteins from their residual envelope genes can protect progeny individuals against infection by viruses that have the same receptor tropism. This may provide a selective advantage that could explain the acquisition and retention of such sequences in evolution (see Section 31.2) 6.2 Infection of animal cells: enveloped viruses The outermost layer of many animal virus particles is a lipid bilayer, or envelope, in which attachment proteins are embedded. These viruses deliver their genetic material and other contents into the cell by membrane fusion, either at the cell surface or after being taken into the cell in an endocytic vesicle. The lipid envelope and embedded proteins become part of the cell membrane. The attachment proteins of enveloped viruses are transmembrane proteins that span the envelope bilayer and are usually multimeric assemblies, meaning each attachment protein is multivalent.
eBook - PDF- Marc H.V. van Regenmortel, Brian W.J. Mahy(Authors)
- 2010(Publication Date)
- Academic Press(Publisher)
Expert Reviews in Molecular Medicine 6: 1–18, Cambridge University Press. Gonzalez ME and Carrasco L (2003) Viroporins. FEBS Letters 552: 28–34. Harrison SC (2006) Principles of virus structure. In: Knipe DM, Howley PM, Griffin DE, et al. (eds.) Fields Virology, 5th edn, pp. 59–98. Philadelphia: Lippincott Williams and Wilkins. Jardetzky TS and Lamb RA (2004) A class act. Nature 427: 307–308. Kielian M and Rey FA (2006) Virus membrane fusion proteins: More than one way to make a hairpin. Nature Reviews Microbiology 4: 67–76. Simons K and Vaz WLC (2004) Model systems, lipid rafts and cell membranes. Annual Review of Biophysics and Biomolecular Structure 33: 269–295. Smith AE and Helenius A (2004) How viruses enter animal cells. Science 304: 237–242. O u tside cell Inside cell Lipid (b) (c) (a) RNP RNP RNP Figure 4 One kind of virus budding. Viral glycoproteins, inserted into the cellular membrane at the endoplasmic reticulum and processed through the Golgi to the plasma membrane (see Figure 3 ), associate with the assembled viral nucleocapsid. The direct association pictured here is characteristic of togaviruses. For other viruses, possessing helical nucleocapsids, the association is mediated by a peripheral membrane protein. Cellular membrane proteins are excluded from the envelope of the mature virion. This may occur during assembly, as pictures, or by prior formation of a viral membrane patch (or raft), before the nucleocapsid arrives at the membrane. Viral Membranes 187 Mimivirus J-M Claverie, Universite ´ de la Me ´ diterrane ´ e, Marseille, France ã 2008 Elsevier Ltd. All rights reserved. Glossary 16S rDNA The gene encoding the small ribosomal RNA molecule, a universal component of all cellular prokaryotic organisms, the sequence of which is used for identification and classification purposes. Aminoacyl-tRNA synthetases The highly specific enzymes responsible for the loading of a given amino acid onto its cognate tRNA(s).
eBook - PDFPathobiology of Cell Membranes
Volume II
- Benjamin F. Trump, Antti U. Arstila, Benjamin F. Trump, Antti U. Arstila(Authors)
- 2013(Publication Date)
- Academic Press(Publisher)
Intramembranous portions of viro-receptors have been visualized as 75 Â diameter particulates within freeze-cleaved surfaces where they appear to fit into the lipid mosaic. Various interesting chemical spe-cificities which can influence the virus binding, even in cell-free systems, are being studied and may shed much more light on normal informational molecule interactions. Ultrastructural studies of some viruses of the adenovirus class suggest morphological identification of antenna-like areas which represent the binding portion of the virion. The long controversial question of membrane fusion can also be ap-proached through the study of viruses as these represent important tools for studying membrane fusion. Fusion of cells into syncytia is also an important concomitant of systemic virus infections in man. Viruses produce factors which stimulate membrane fusion, explaining some of the multicellular syn-cytia seen in certain virus infections. Amazingly, several studies indicate that certain virus envelope proteins can be actually inserted into the host cell EDITORS SUMMARY TO CHAPTER III 171 membrane and diffuse within the membrane within a few minutes after attachment. This leads to instant changes in the characteristics of the host cell surface membrane. Such mechanisms may also relate to the mechanism involved in membrane fusion. There are a number of membrane-dependent steps in virus-cell interac-tions which are important both to the virus cycle and also to the host cell response. Many of these are related to modification of host cell membrane synthesis and often marked proliferation of cytomembranes resembling smooth endoplasmic reticulum are seen. Such new membrane synthesis in the host cell may be fairly analogous to stimulation of membrane prolifera-tion by chemicals. In togavirus infections the membrane spherules, possibly derivatives of phagocytic vacuoles may participate directly in togaviral RNA replication.
eBook - PDFMembrane Fluidity in Biology
Cellular Aspects
- Roland C. Aloia, Joan M. Boggs, Roland C. Aloia, Joan M. Boggs(Authors)
- 2013(Publication Date)
- Academic Press(Publisher)
The extent of exocytosis, the number of vesicles, and the amount of membrane undergoing fusion must be regulated precisely, and only specific stimuli cause release. The biochemical processes providing this specificity are in-completely understood, but are regulated in part by protein interactions with the membrane. The fusion of these biologic membranes must be regulated precisely in time, extent, cellular location, and specificity of stimuli. Membrane proteins are intimately involved in the regulation and mediation of this fusion process. Role of Proteins in Membrane Fusion 261 II. Role of Viral Envelope Proteins in Virus-Host Cell Fusion During infection of cells by enveloped viruses, the virus enters the cell and then loses its lipid envelope (Lenard and Miller, 1982). Viruses may enter the cell in one of two ways. The viral envelope may fuse with the plasma membrane of the cell and thereby introduce the viral nucleoprotein into the cytoplasm. Alternatively, the virus may penetrate the cell by adsorptive endocytosis and fuse with the membrane of the endocytotic vesicle, a pro-cess called viropexis. The entry of virus by either fusion or viropexis is determined both by the type of virus and by the environment of the virus-host cell interaction, particularly the pH. Fusion of the enveloped virus with the plasma membrane of the host cell is mediated by protein in the virus coat. The role of virus envelope proteins in fusion has been extensively studied in the orthomyxovirus (influenza), the paramyxovirus (measles, mumps, Sendai, and Newcastle disease), and group A arbovirus (Semliki Forest virus). Of these viruses, the mechanisms of fusion of influenza and Sendai have been most extensively studied. A. ORTHOMYXOVIRUS FUSION The orthomyxovirus influenza is an enveloped virus (Klenk et al., 1979).
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