Another strategy used by viruses to counteract the action of antibodies and other entry inhibitors is the direct cell-to-cell transmission of viruses, which avoids or reduces the exposure of the virus to inhibitors that are not designed to penetrate through membranes or other barriers. This could be one important reason for the low efficacy that is observed in vivo of otherwise potent in vitro inhibitors.
Low inhibitor potency combined with a high virus mutation rate is perhaps the most challenging problem in the development of entry inhibitors.
Although the rapid progress in our understanding of the structural mechanisms of virus entry promises new approaches that could 'outsmart' the virus, no entry inhibitors or treatment protocols in clinical use have so far been developed on the basis of predictions made by structural models, and the main source of new inhibitors is still from screening large libraries of small organic molecules, natural products, peptides and antibodies.
However, structures of entry proteins have been invaluable for the development of our understanding of the mechanisms of inhibition and should allow further improvement of the inhibitors. It seems likely that sooner or later structure-based design will yield entry inhibitors that will be in clinical use.
Conserved entry intermediates as targets for inhibitors; multivalent inhibitors and other approaches. One direction of research that could hasten the arrival of entry inhibitors to the clinics is the development of compounds that interact with conserved intermediates of the entry process or with the protein structures that, on binding to receptors, trigger the conformational changes that lead to the formation of these entry intermediates.
Typically, such intermediates are only transiently exposed, so viruses might not have evolved strategies to avoid inhibitors targeted to these structures.
In addition, conserved intermediates are usually important for virus entry and presumably cannot easily be substituted by other structures after mutation. An example of the successful design of an entry inhibitor that shows proof of the concept is the 5-Helix protein, which interferes with a conserved intermediate in the entry of HIV-1 Ref. A related example is a class of peptides that could have broad applications to several viruses containing class I fusion proteins.
The peptides are derived from regions of fusion proteins heptad repeats that have a propensity to form coiled-coils and which serve as fusion intermediates and enable oligomerization of proteins. T also known as DP is derived from the carboxy-terminal heptad region of the HIV-1 gp41 and showed potent inhibitory activity in vivo Bossart and C. Broder, personal communication , perhaps owing to the endocytic entry route of these viruses. Although peptides have certain promising features, including a relatively small size that might ensure good penetration combined with high binding affinity, the lack of oral formulations, short half-life, possible toxicity and immunogenicity might limit their application.
Recently, a small molecule, BMS, was identified that inhibits the entry of a broad range of HIV-1 isolates by a mechanism which was attributed to competition with the CD4 receptor for binding to gp Ref. So, at inhibitory concentrations, BMS does not interfere with CD4 binding, indicating a different inhibitory mechanism.
By analogy with inhibitors of picornavirus entry, such as pleconaril, BMS might inhibit the entry of HIV-1 by binding to conserved structures that are important for the conformational changes which gp must undergo for viral entry Potent virus-specific inhibitors of the viral-membrane-merging step have not been identified yet. Another promising direction is the development of multivalent inhibitors that can overcome problems caused by mutation of viral proteins to escape inhibition because multivalent inhibitors bind to several regions of the same or different protein s on the viral surface.
One example of a multivalent inhibitor is the multimeric soluble receptors of influenza and HIV-1 Ref. The use of entry inhibitors in combination or as fusion proteins could also result in increased efficiency. Finally, improvement of current methods for structure-based design by accounting for protein flexibility and dynamics in binding to ligands 85 , and screening methods for inhibitors 86 , 87 would certainly expand the range of possible inhibitors that can be tested.
Neutralizing antibodies and vaccine immunogen design. Neutralizing antibodies usually inhibit virus entry by preventing attachment of the virus to the cell or by binding to entry intermediates 88 , 89 , Human immunoglobulin composed of concentrated antibodies collected from pooled human plasma has been successfully used as a preventative treatment for virus infections, including rabies, hepatitis A and B, measles, mumps, varicella, cytomegalovirus and arenaviruses.
Antibodies can completely prevent infection, but once infection is established they are a much less efficient treatment. The only monoclonal antibody in clinical use today to treat a viral disease — Synagis MEDI — is more potent than the polyclonal immunoglobulin that is presently in use, and is broadly active against numerous RSV type A and B clinical isolates It binds to the F protein of RSV with high affinity 3 nM and inhibits virus entry and cell fusion in vitro with an IC 50 of approximately 0.
It seems that the efficacy of Synagis in vivo is correlated with the high affinity of binding and potency of this antibody in vitro The X-ray crystal structures of rhinovirus 21 and poliovirus 22 indicate a possible mechanism by which picornaviruses can avoid neutralization by antibodies through the mutation of non-conserved amino acid residues surrounding the receptor-binding site — a 2 nm deep and 2 nm wide canyon Fig.
It was initially hypothesized that the conserved amino acid residues of the canyon are not accessible by antibodies; however, it was later shown that a strongly neutralizing antibody, Fab17, can penetrate deep within the receptor-binding canyon by undergoing a large conformational change without inducing conformational changes in the virus 90 , Unusually, not only the hypervariable residues but also residues from the framework region of Fab17 contact the canyon.
Yet another remarkable mechanism of immune recognition of viruses is the recently discovered receptor mimicry by post-translational modification tyrosine sulphation of antibodies Ref. It seems that any accessible viral surface can be recognized by antibodies. Rapidly mutating viruses can escape neutralizing antibodies even if they bind to structures that are essential for virus replication, such as receptor-binding sites, unless they bind with energetically identical profiles Whether a virus will escape neutralization by antibodies depends on the interplay between the antibody affinity avidity and kinetics of binding, generation rate, concentration and the viral mutation rate and fitness.
Mutations of immunodominant structural loops that form antibody-binding sites and mutations leading to changes in oligosaccharide attachment to viral entry proteins are common mechanisms by which viruses avoid neutralization 90 , Mutations of conserved residues that have a role in the entry mechanism typically result in reduction or loss of infectivity. Antibodies or their derivatives that bind to epitopes where residues contribute most of the binding energy could have potential as entry inhibitors.
Epitopes that are exposed after virus binding to receptors are typically well conserved — for example, the 17b 39 and X5 Ref. One potential problem with using antibodies as entry inhibitors in this case could be limited access to the post-receptor-binding state of the viral entry protein due to the relatively large size of the whole antibody. Solving this, and other problems, could lead to the development of potent broadly neutralizing antibodies which could limit the generation of resistant viruses, especially if these inhibitors are used in combination with other antibodies or inhibitory molecules.
Many viruses, especially RNA viruses such as HIV-1, exist as swarms of virions inside an infected individual, and might significantly differ in sequence between isolates. So, elicitation of potent, broadly neutralizing antibodies is an important goal for vaccine development.
However, elicitation of these antibodies in vivo has not been successful. Identification of broadly neutralizing antibodies and the characterization of their epitopes could help to design vaccine immunogens that would be able to elicit these neutralizing antibodies in vivo — so-called retrovaccinology At present, all vaccines that elicit antibodies against entry proteins have been developed empirically using an antigen, rather than by designing an immunogen on the basis of the antibodies produced.
The important advantages of human antibodies as therapeutics are low or negligible toxicity combined with high potency and a long half-life. However, drawbacks include the generation of neutralization-resistant virus mutants, limited access of the large antibody molecules to the site of virus replication, lack of oral formulations and the high cost of production and storage.
Viruses are usually associated with disease. However, some viruses can be beneficial. The HERV-W Env, known as syncytin, is fusogenic and has a role in human trophoblast cell fusion and differentiation Retroviral particles have been observed in the placenta, along with fused placental cells, which are morphologically reminiscent of virally induced syncytia.
These studies led to the proposal that an ancient retroviral infection might have been a pivotal event in mammalian evolution Viruses have long been used to transfer genes into cells. During the last decade, another important application has been the viral delivery of genes and drugs to treat cancer. A major challenge has been to develop virus entry proteins to deliver molecules to specific cells with high efficiency.
To achieve this goal it is often desirable to engineer viruses that do not infect cells expressing the native receptor, but instead target a cell of choice. Engineering of entry proteins in this way is known as transductional retargeting A conceptually simple approach to transductional retargeting is to incorporate the protein that determines cell tropism into the infecting virion of choice — known as virus 'pseudotyping'. This has been used in both retroviruses and adenoviruses, and does not require prior knowledge of specific virus—receptor interactions.
In a related approach, viral entry proteins are used to produce drug and gene delivery vehicles, for example, the F protein of Sendai virus has been incorporated into liposomes to form virosomes and the L protein of hepatitis B has been incorporated into yeast-derived lipid vesicles Retargeting of retroviruses, adenoviruses and AAVs has been achieved by conjugation of entry proteins with molecular adaptors, such as bi-specific antibodies that have particular receptor-binding properties.
Modification of the entry proteins so that the normal receptor-binding property is abolished, or a ligand for alternative receptor binding is incorporated has also been successful at redirecting adenovirus tropism in cell culture, but is unlikely to work for the entry of viruses that require receptor-induced conformational changes, such as retroviruses, unless detailed molecular mechanisms of those conformational changes are better understood.
A related approach is based on screening libraries of chimaeric Envs from different strains of MLV , or randomized peptides inserted at tolerant sites in viral proteins, such as VP3 of AAV This approach seems promising for the selection of specific retargeting vectors. Understanding the structure of AAV and other viruses could help to further improve the specificity and efficiency of retargeting. Retargeting viruses with complex entry mechanisms that involve several proteins, such as those of herpes viruses and poxviruses, remains challenging.
Elucidation of the molecular mechanisms and the dynamics of the conformational changes driving virus entry remains a significant challenge. It requires the development of new approaches to study the rapid conformational changes of a small number of membrane-interacting protein molecules that are surrounded by many more non-interacting molecules.
A more realistic goal is the determination of the structures of proteins that mediate the entry of all human viruses and the identification of the cognate cellular receptors. If research continues at the present pace, this goal could be accomplished within the next decade.
Identification of all the cellular receptors for human viruses would be an important contribution to our understanding of virus tropism and pathogenesis.
The various, and in many cases unexpected, ways that entry proteins can affect pathogenesis could offer new opportunities for intervention. The development of panels of human monoclonal antibodies against every entry-related protein of all pathogenic human viruses could accelerate our understanding of entry mechanisms and help to fight viral diseases. Recent progress in virus retargeting also raises hopes for the possibility of designing entry machines that can deliver genes and other molecules to any cell of choice.
Google Scholar. Sieczkarski, S. Dissecting virus entry via endocytosis. Pelkmans, L. Local actin polymerization and dynamin recruitment in SVinduced internalization of caveolae. Science , — Dimitrov, D. Cell biology of virus entry. Cell , — Rawat, S. Modulation of entry of enveloped viruses by cholesterol and sphingolipids. Takeda, M. Influenza virus hemagglutinin concentrates in lipid raft microdomains for efficient viral fusion. Natl Acad. USA , — Waarts, B. Lack of correlation with lipid raft formation in target liposomes.
Kielian, M. Specific roles for lipids in virus fusion and exit. Igakura, T. Bomsel, M. Entry of viruses through the epithelial barrier: pathogenic trickery. Nature Rev. Cell Biol. Seisenberger, G. Real-time single-molecule imaging of the infection pathway of an adeno-associated virus.
Lowy, R. Observation of single influenza virus-cell fusion and measurement by fluorescence video microscopy. USA 87 , — Lakadamyali, M. Visualizing infection of individual influenza viruses. Virology , — Quantitation of HIV-1 infection kinetics.
White, J. Fusion of Semliki forest virus with the plasma membrane can be induced by low pH. Shows that a low pH can trigger rapid and efficient fusion of SFV with plasma membranes, which leads to delivery of the viral genome in a form that is suitable for replication. Carr, C. Influenza hemagglutinin is spring-loaded by a metastable native conformation. USA 94 , — Proposes that the native structure of HA is trapped in a metastable state and that the fusogenic conformation is released by destabilization of the native structure.
Hogle, J. Poliovirus cell entry: common structural themes in viral cell entry pathways. Stubbs, M. Anthrax X-rayed: new opportunities for biodefence. Trends Pharmacol. Chen, Y. SNARE-mediated membrane fusion. Rossmann, M. Structure of a human common cold virus and functional relationship to other picornaviruses. Nature , — Three-dimensional structure of poliovirus at 2. References 21 and 22 describe the first crystal structures of human viruses with important implications for understanding their mechanisms of entry and design of inhibitors.
Mancini, E. Cryo-electron microscopy reveals the functional organization of an enveloped virus, Semliki Forest virus. Cell 5 , — Kuhn, R. Structure of dengue virus: implications for flavivirus organization, maturation, and fusion.
Chappell, J. EMBO J. Structural analysis reveals evolutionary relationships between two unrelated virus families.
Wilson, I. The first, and still the most informative structure of an entry envelope glycoprotein — the influenza HA. Chen, L. The structure of the fusion glycoprotein of Newcastle disease virus suggests a novel paradigm for the molecular mechanism of membrane fusion. Structure 9 , — Rey, F. The first structure of a class II fusion protein that reveals an entirely unexpected configuration that is dramatically different from the structure of class I fusion proteins.
Modis, Y. A ligand-binding pocket in the dengue virus envelope glycoprotein. In plants used for landscaping, two of the most common viruses are peony ring spot and rose mosaic virus. There are far too many plant viruses to discuss each in detail, but symptoms of bean common mosaic virus result in lowered bean production and stunted, unproductive plants.
In the ornamental rose, the rose mosaic disease causes wavy yellow lines and colored splotches on the leaves of the plant. Privacy Policy. Skip to main content. Search for:. Virus Infections and Hosts. Steps of Virus Infections Viral infection involves the incorporation of viral DNA into a host cell, replication of that material, and the release of the new viruses. Learning Objectives List the steps of viral replication and explain what occurs at each step. Key Takeaways Key Points Viral replication involves six steps: attachment, penetration, uncoating, replication, assembly, and release.
During attachment and penetration, the virus attaches itself to a host cell and injects its genetic material into it. During release, the newly-created viruses are released from the host cell, either by causing the cell to break apart, waiting for the cell to die, or by budding off through the cell membrane. Key Terms virion : a single individual particle of a virus the viral equivalent of a cell glycoprotein : a protein with covalently-bonded carbohydrates retrovirus : a virus that has a genome consisting of RNA.
The Lytic and Lysogenic Cycles of Bacteriophages Bacteriophages, viruses that infect bacteria, may undergo a lytic or lysogenic cycle.
Learning Objectives Describe the lytic and lysogenic cycles of bacteriophages. Key Takeaways Key Points Viruses are species specific, but almost every species on Earth can be affected by some form of virus. The lytic cycle involves the reproduction of viruses using a host cell to manufacture more viruses; the viruses then burst out of the cell.
The lysogenic cycle involves the incorporation of the viral genome into the host cell genome, infecting it from within. Key Terms latency : The ability of a pathogenic virus to lie dormant within a cell.
Animal Viruses Animal viruses have their genetic material copied by a host cell after which they are released into the environment to cause disease.
Learning Objectives Describe various animal viruses and the diseases they cause. Key Takeaways Key Points Animal viruses may enter a host cell by either receptor -mediated endocytosis or by changing shape and entering the cell through the cell membrane. Viruses cause diseases in humans and other animals; they often have to run their course before symptoms disappear. Examples of viral animal diseases include hepatitis C, chicken pox, and shingles.
Key Terms receptor-mediated endocytosis : a process by which cells internalize molecules endocytosis by the inward budding of plasma membrane vesicles containing proteins with receptor sites specific to the molecules being internalized. Plant Viruses Plant viruses can cause damage to stems, leaves, and fruits and can have a major impact on the economy because of food supply disruptions.
Learning Objectives Give examples of plant viruses and explain why they can be costly to the economy. Key Takeaways Key Points Plants have cell walls which protect them from viruses entering their cells, so some type of damage must occur in order for them to become infected. When viruses are passed between plants, it is called horizontal transmission; when they are passed from the parent plant to the offspring, it is called vertical transmission. Symptoms of plant virus infection include malformed leaves, black streaks on the stems, discoloration of the leaves and fruits, and ring spots.
Plant viruses can cause major disruptions to crop growth, which in turn can have a major impact on the economy. Key Terms horizontal transmission : the transmission of an infectious agent, such as bacterial, fungal, or viral infection, between members of the same species that are not in a parent-child relationship vertical transmission : the transmission of an infection or other disease from the female of the species to the offspring. Licenses and Attributions.
Some remain outside the cytoplasm and have to double their genetic material without help. A special nanomachine combined with various subunits — RNA polymerase — plays an important role in this process. This process is called transcription. This pathogen, which is completely harmless to humans, is not only the basis for all vaccines against smallpox infections.
It is also tested in oncolytic virotherapy to combat cancer. The core enzyme is similar to the molecular copy machine that occurs naturally in living cells and is the subject of intensive research by the Cramer Department: RNA polymerase II. The second complex of Vaccinia RNA polymerase is an all-rounder. It consists of numerous subunits and carries out the entire transcription process for the virus.
This enables the virus to multiply. This type of molecule does not normally play a role in transcription, but provides the amino acid building blocks for protein production. If this host tRNA were not involved, the huge complex would likely fall apart. Structures of Vaccinia transcription complexes purified from infected cells.
The individual subunits are shown in different colors. In addition to the core Vaccinia RNA polymerase, it contains viral transcription factors required for the different steps of the transcription cycle as well as a host tRNA molecule shown as cartoon. To find out how the viral RNA polymerase works, the researchers determined — under the participation of Henning Urlaub, Research Group Leader at the MPI for Biophysical Chemistry — its three-dimensional structure during different transcription steps.
This is done by inserting virus genetic material into a host cell. This causes the cell to make a copy of the virus DNA, making more viruses. Many scientists argue that even though viruses can use other cells to reproduce itself, viruses are still not considered alive under this category. This is because viruses do not have the tools to replicate their genetic material themselves.
More recently, scientists have discovered a new type of virus, called a mimivirus. These viruses do contain the tools for making a copy of its DNA. This suggests that certain types of viruses may actually be living. Viruses only become active when they come into contact with a host cell. Image by CarlosRoBe. Living things use energy. Outside of a host cell, viruses do not use any energy. They only become active when they come into contact with a host cell. Because they do not use their own energy, some scientists do not consider them alive.
This is a bit of an odd distinction though, because some bacteria rely on energy from their host, and yet they are considered alive. These types of bacteria are called obligate intracellular parasites. Living things respond to their environment.
Whether or not viruses really respond to the environment is a subject of debate. They interact with the cells they infect, but most of this is simply based on virus anatomy.
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