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The Sun, like most stars in the Universe, is on the main sequence stage of its life, during which nuclear fusion reactions in its core fuse hydrogen into helium. Every second, 600 million tons of matter are converted into neutrinos, solar radiation, and roughly 4 x 1027 Watts of energy. For the Sun, this process began 4.57 billion years ago, and it has been generating energy this way every since.
This places more pressure on the core, which is resisted by a resulting increase in the rate at which fusion occurs. Basically, this means that as the Sun continues to expend hydrogen in its core, the fusion process speeds up and the output of the Sun increases. At present, this is leading to a 1% increase in luminosity every 100 million years, and a 30% increase over the course of the last 4.5 billion years.
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Some CoVs were originally found as enzootic infections, limited only to their natural animal hosts, but have crossed the animal-human species barrier and progressed to establish zoonotic diseases in humans [19,20,21,22,23]. Accordingly, these cross-species barrier jumps allowed CoVs like the SARS-CoV and Middle Eastern respiratory syndrome (MERS)-CoV to manifest as virulent human viruses. The consequent outbreak of SARS in 2003 led to a near pandemic with 8096 cases and 774 deaths reported worldwide, resulting in a fatality rate of 9.6% . Since the outbreak of MERS in April 2012 up until October 2018, 2229 laboratory-confirmed cases have been reported globally, including 791 associated deaths with a case-fatality rate of 35.5% . Clearly, the seriousness of these infections and the lack of effective, licensed treatments for CoV infections underpin the need for a more detailed and comprehensive understanding of coronaviral molecular biology, with a specific focus on both their structural proteins as well as their accessory proteins [26,27,28,29,30]. Live, attenuated vaccines and fusion inhibitors have proven promising, but both also require an intimate knowledge of CoV molecular biology [29, 31,32,33,34,35,36].
The coronaviral genome encodes four major structural proteins: the spike (S) protein, nucleocapsid (N) protein, membrane (M) protein, and the envelope (E) protein, all of which are required to produce a structurally complete viral particle [29, 37, 38]. More recently, however, it has become clear that some CoVs do not require the full ensemble of structural proteins to form a complete, infectious virion, suggesting that some structural proteins might be dispensable or that these CoVs might encode additional proteins with overlapping compensatory functions [35, 37, 39,40,41,42]. Individually, each protein primarily plays a role in the structure of the virus particle, but they are also involved in other aspects of the replication cycle. The S protein mediates attachment of the virus to the host cell surface receptors and subsequent fusion between the viral and host cell membranes to facilitate viral entry into the host cell [42,43,44]. In some CoVs, the expression of S at the cell membrane can also mediate cell-cell fusion between infected and adjacent, uninfected cells. This formation of giant, multinucleated cells, or syncytia, has been proposed as a strategy to allow direct spreading of the virus between cells, subverting virus-neutralising antibodies [45,46,47].
Copyright: 2010 Rebecca Ellis Dutch. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Paramyxoviruses are a family of non-segmented RNA viruses that includes major human pathogens such as measles virus and respiratory syncytial virus (RSV) and significant animal viruses like rinderpest . In recent years, several new paramyxoviruses have been identified, further increasing the breadth and importance of this viral family. While many elements of the fusion and entry mechanisms of these recently identified pathogens are conserved, there are interesting differences, including variations in receptor binding, cell tropism, fusion (F) protein proteolytic activation, and triggering of membrane fusion. Thus, study of their entry mechanisms has highlighted the diversity of these critical events in the family.
To enter host cells, paramyxoviruses must go through the key steps of viral attachment to the target cell, followed by fusion of the viral membrane to a host cell membrane . Two major viral glycoproteins promote these events: the attachment protein facilitates primary receptor binding of the virus to the target cell, while the F protein promotes the subsequent membrane fusion events. Both events are hypothesized to occur at the cell surface in a neutral pH environment. Interactions between the F protein and the homotypic attachment protein are hypothesized to control initiation of the fusion process for most paramyxoviruses, though the mechanistic details of triggering control remain elusive. Once begun, fusion is promoted by a series of conformational changes in the F protein that first lead to insertion of a hydrophobic region (termed the fusion peptide) into the target membrane, forming a protein bridge between the two membranes. Additional conformational changes lead to formation of a helical bundle, formed by interactions between two heptad repeat regions that do not interact in the prefusion form of the protein , and subsequent membrane fusion.
A number of factors point to an overall conserved mechanism of fusion promotion among the paramyxovirus F proteins. While there is considerable heterogeneity at the amino acid level, F proteins from both established and newly identified paramyxoviruses display conserved positioning of cysteine, glycine, and proline residues, suggesting an overall conservation of structure. F proteins also contain similarly placed fusion peptide and heptad repeat regions. Peptides corresponding to the F protein heptad repeat regions have been shown to block fusion and entry for previously studied paramyxoviruses, and similar peptides inhibit Hendra, Nipah, and HMPV fusion and entry, indicating that the requirement for formation of the final helical bundle is a conserved feature , . Like previously identified members of the family, fusion activity of the Hendra and Nipah F proteins requires the presence of a viral attachment protein, though either the Hendra or Nipah attachment protein can be used interchangeably . As was seen with measles virus, recent evidence suggests that fusion activity for the Hendra and Nipah F proteins is inversely proportional to the strength of the F attachment protein interactions, in contrast to results from other paramyxovirus systems such as Newcastle disease viruses , suggesting slightly different mechanisms of control of fusion initiation.
Like other paramyxovirus F proteins, the Hendra and Nipah virus F proteins are initially synthesized as a precursor (F0) that must be proteolytically processed to two subunits (F1 and F2) to be fusogenically active (Figure 2A). For the majority of F proteins, this critical proteolytic processing event is promoted by furin, a cellular protease present primarily in the trans-Golgi network. Interestingly, the mechanism for proteolytic activation of the henipavirus F proteins is completely novel. Furin is clearly not involved, as there is no furin consensus at the cleavage site, furin inhibitors have no effect on henipavirus F processing, and processing occurs efficiently in furin-negative cell lines . Instead, inhibitors or shRNA knock-downs of the cellular endosomal protease cathepsin L were shown to inhibit cleavage of the Hendra and Nipah F proteins, and in vitro studies confirmed proteolytic cleavage of the henipavirus F proteins at a single specific site by purified cathepsin L , . To facilitate this key interaction with cathepsin L, endocytosis of the Hendra F protein  and the Nipah F protein  must occur, followed by a retrafficking event to the cell surface after proteolytic processing (Figure 2B). As cleaved F protein is present within the packaged virion , this complex trafficking of the henipavirus F proteins through the endosomal pathway occurs prior to viral assembly. Interestingly, the Hendra G attachment protein does not follow this complicated trafficking pathway, indicating that the critical attachment protein: fusion protein interactions needed for fusion occur only after F protein endocytic trafficking and proteolytic cleavage . The reason for this novel activation pathway is unclear, though it is intriguing to note that Ebola virus and SARS coronavirus also have a role for cathepsin L at some point during the viral life cycle, and like Hendra and Nipah virus, the reservoir species for these important pathogens is thought to be bats. Future studies on protease profiles in bat cells may shed light on the reason for the unusual role of cathepsins in the life cycles of these pathogens. 1e1e36