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.
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Some bacteriophages, such as Enterobacteria phage T4, have a complex structure consisting of an icosahedral head bound to a helical tail, which may have a hexagonal base plate with protruding protein tail fibers. This tail structure acts like a molecular syringe, attaching to the bacterial host and then injecting the viral genome into the cell. T4 Bacteriophage : T4 is a bacteriophage that infects E. Although it has an icosahedral head, its tail makes it asymmetrical, or complex in terms of structure.
The poxviruses are large, complex viruses that have an unusual morphology. The viral genome is associated with proteins within a central disk structure known as a nucleoid. The nucleoid is surrounded by a membrane and two lateral bodies of unknown function.
The virus has an outer envelope with a thick layer of protein studded over its surface. The whole virion is slightly pleiomorphic, ranging from ovoid to brick shape.
Mimivirus is the largest characterized virus, with a capsid diameter of nm. Protein filaments measuring nm project from the surface. The capsid appears hexagonal under an electron microscope, therefore the capsid is probably icosahedral. In , researchers discovered a larger virus on the ocean floor off the coast of Las Cruces, Chile. Provisionally named Megavirus chilensis , it can be seen with a basic optical microscope.
Some viruses that infect archaea have complex structures unrelated to any other form of virus. These include a wide variety of unusual shapes, ranging from spindle-shaped structures, to viruses that resemble hooked rods, teardrops, or even bottles.
Other archaeal viruses resemble the tailed bacteriophages, and can have multiple tail structures. Privacy Policy. Skip to main content. Search for:. Structure of Viruses Viral Morphology Viruses of all shapes and sizes consist of a nucleic acid core, an outer protein coating or capsid, and sometimes an outer envelope.
Learning Objectives Describe the relationship between the viral genome, capsid, and envelope. Key Takeaways Key Points Viruses are classified into four groups based on shape: filamentous, isometric or icosahedral , enveloped, and head and tail. Many viruses attach to their host cells to facilitate penetration of the cell membrane, allowing their replication inside the cell. Non-enveloped viruses can be more resistant to changes in temperature, pH, and some disinfectants than are enveloped viruses.
Animal viruses, such as HIV, are frequently enveloped. Head and tail viruses infect bacteria. They have a head that is similar to icosahedral viruses and a tail shape like filamentous viruses. Many viruses use some sort of glycoprotein to attach to their host cells via molecules on the cell called viral receptors. For these viruses, attachment is a requirement for later penetration of the cell membrane, allowing them to complete their replication inside the cell. The receptors that viruses use are molecules that are normally found on cell surfaces and have their own physiological functions.
Viruses have simply evolved to make use of these molecules for their own replication. Overall, the shape of the virion and the presence or absence of an envelope tell us little about what disease the virus may cause or what species it might infect, but they are still useful means to begin viral classification.
Among the most complex virions known, the T4 bacteriophage, which infects the Escherichia coli bacterium, has a tail structure that the virus uses to attach to host cells and a head structure that houses its DNA. Adenovirus, a non-enveloped animal virus that causes respiratory illnesses in humans, uses glycoprotein spikes protruding from its capsomeres to attach to host cells.
Non-enveloped viruses also include those that cause polio poliovirus , plantar warts papillomavirus , and hepatitis A hepatitis A virus.
Enveloped virions like HIV consist of nucleic acid and capsid proteins surrounded by a phospholipid bilayer envelope and its associated proteins. Study: What are the factors that influence the reach of single-use surface disinfection wipes?
New manual reprocessing procedure for wipes dispensers. Expert interview on biofilm-developing gram-negative pathogens. Preventing Clostridium difficile infections. Relevant pathogens from A-Z.
Reprocessing of Medical Devices.
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