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Introduction to Cells, DNA, and Viruses
Published in Patricia G. Melloy, Viruses and Society, 2023
Now if we think about viruses for a moment, without the chainsaw, we can see that they exist in this unusual space in the world in that they do not exhibit all the characteristics of life, yet they certainly seem to be alive in their ability to enter and use cells. Upon critical examination, viruses are not organized into cells, and they cannot reproduce by themselves (Minkoff and Baker 2004a). For this reason, most scientists have settled on the idea of viruses as obligate intracellular parasites, which display many characteristics of life but are not living themselves (Summers 2009; Cossart and Helenius 2014). This definition indicates that viruses need to exist inside cells to perpetuate themselves. Virologists, scientists who study viruses, have created an even more specific list of viral characteristics based on the genes viruses must carry and how they assemble themselves. The characteristics include that a virus must have a gene with the instructions for making at least one capsomere (also called capsomer) protein that is a part of the coat protecting the viral genome (called the capsid). In addition, the virus can build itself after new viral proteins are made in the cell and the genome is copied. Finally, importantly, viruses are capable of evolution, like the host they infect is (Lostroh 2019).
The Viruses
Published in Julius P. Kreier, Infection, Resistance, and Immunity, 2022
The structural proteins surrounding viral genomes are arranged into one of two symmetrical forms called capsids that are either helical or icosahedral in shape. The simplest viruses consist of a rodlike helix or coil of RNA closely associated with structural proteins. There are no known animal viruses lacking an outer envelope and thus naked helical morphology; however, an example of one found in plants is the tobacco mosaic virus. The simplest animal viruses are naked icosahedral viruses such as the parvoviruses. They consist of a DNA or RNA strand within a protein shell called a capsid (Figure 16.1 A). The capsid consists of a structure created by the regular arrangement of structural subunits called “capsomeres.” Each capsomer is composed of a set of viral structural proteins. The other major forms of viruses are the enveloped icosahedral viruses such as the herpesviruses or the enveloped helical viruses such as the rhabdoviruses (Figure 16.1 B and 16.1C). Viral nucleic acid strands with bound proteins generally have helical morphology while viral genomes within a capsid structure also referred to as a nucleocapsid are characteristically icosahedral in morphology.
3D Particles
Published in Paul Pumpens, Single-Stranded RNA Phages, 2020
Finally, Dent et al. (2013) demonstrated the asymmetric structure of the phage MS2, attached to its receptor, the F pilus, by electron cryotomography. The subtomographic averaging of such complexes resulted in a structure containing clear density for the packaged genome, implying that the conformation of the genome was the same in each virus particle. The data also suggested for the first time that the single-copy viral maturation protein broke the symmetry of the capsid, occupying a position that would be filled by a coat protein dimer in an icosahedral shell. This capsomere could thus fulfill its known biological roles in receptor and genome binding and suggested an exit route for the genome during infection (Dent et al. 2013). Figure 21.8 demonstrates how the maturation protein replaces the coat dimer in the MS2 virion. Figure 21.9 is invented to show high potential of the electron cryotomography approach. It demonstrates fantastic quality of the image of the MS2-decorated F pili (Dent et al. 2013). Summarizing, this study made it possible for the first time to overcome the traditional oversimplified presentation of the RNA phage as a perfect icosahedral assembly.
Novel strategies for the development of hand, foot, and mouth disease vaccines and antiviral therapies
Published in Expert Opinion on Drug Discovery, 2022
Similar to antiviral drugs, all steps of the virus replication cycle, including attachment, entry, uncoating, viral RNA translation and polyprotein processing, RNA replication, and virion assembly and release, can be targeted for the development of antiviral aptamers. There are several aptamers developed for the treatment of viral infection by the SELEX method (reviewed in [141]). The targets of these antiviral aptamers include envelope/capsid proteins, viral proteases, and viral nucleic acid polymerases. Some of these aptamers have been proven to have antiviral and protective reaction in animal studies [142], but none of them have proceeded to clinical trials. Currently, there are two enterovirus studies engaging the use of aptamers. Chandler-Bostock et al. have reported the identification of multiple RNA sequences across the genome of enterovirus-E that revealed affinity for the cognate viral capsid protein capsomer. In this study, RNA SELEX was used to identify the RNA-binding motifs (aptamers) for these capsid proteins [143]. In another preprint manuscript, Chauhan et al. have identified the conserved enteroviral nucleic acid sequences by gold-aptamer nanoconstructs that are complementary to the target enteroviral RNA sequences [144]. At present, there is no study report the development of antiviral aptamer for enterovirus presently. Given that the SELEX has been applied for the development of various antiviral aptamers, it is anticipated that this method will provide a fast and promising strategy for the construction of antiviral drugs for enteroviruses.
Human papillomavirus and cervical cancer
Published in Journal of Obstetrics and Gynaecology, 2020
HPV is a member of the Papovaviridae family. It is a relatively small, non-enveloped virus of about 55 nm diameter. It has an icosahedral capsid with 72 capsomers and these contain at least two capsid proteins, L1 and L2. Each capsomer is a pentamer of the major capsid protein, L1 (Baker et al. 1991). Each virion capsid contains about 12 copies of the minor capsid protein, L2 (Sapp et al. 1995). The HPV genome consists of a single molecule of double-stranded, circular DNA (Favre 1975) with all open reading frame (ORF) protein-coding sequences confined to one strand. There are three functional regions in the genome (Figure 1, Stanley et al. 2007): the first is a ‘non-coding upstream regulatory region’ also referred to as the long control region (LCR), or the upper regulatory region (URR). This region contains the highest degree of variation in the viral genome and contains the p97 core promoter along with enhancer and silencer sequences that control ORFs transcription in the regulation of DNA replication (Apt et al. 1996). The second is called the ‘early region (E)’ and it consists of ORFs E1, E2, E4, E5, E6 and E7, which are involved in viral replication and tumorigenesis. The third is referred to as the ‘late region (L)’ and this encodes the L1 and L2 ORFs for the viral capsid. The E6, E7 and L1 ORFs of a new or unknown HPV type should be 90% or less homologous to the corresponding sequences of known HPV types (Torrisi et al. 2000).
Natural and vaccine-induced B cell-derived systemic and mucosal humoral immunity to human papillomavirus
Published in Expert Review of Anti-infective Therapy, 2020
Ralph-Sydney Mboumba Bouassa, Hélène Péré, Mohammad-Ali Jenabian, David Veyer, Jean-François Meye, Antoine Touzé, Laurent Bélec
The HPV structural genes encode the major (L1) and minor (L2) proteins that form the nonenveloped icosahedral viral capsid, which comprises 72 pentameric L1 capsomers, and each capsomer has an upper estimate of one L2 protein [47,48]. The L1 protein mediates attachment to host cells, while the L2 protein is essential for subsequent viral infectivity. The L1 protein can spontaneously self-assemble into virus-like particles (VLPs) [49], which are the basis of the current prophylactic HPV vaccines. In addition, the early (E1, E2, E4, E5, E6, E7) proteins interact with host and viral proteins to maintain viral replication. The late proteins form the viral protein coat during productive infections.