https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6537279/
Virol J. 2019; 16: 69.
Published online 2019 May 27. doi: 10.1186/s12985-019-1182-0
PMCID: PMC6537279
PMID: 31133031
( Artikkelin alkuosasta yleistä taustatietoa sitaattina CoV koronaviruksesta ja sen proteiineista S, M, N, E ja lisäproteiineista)
( Artikkelin alkuosasta yleistä taustatietoa sitaattina CoV koronaviruksesta ja sen proteiineista S, M, N, E ja lisäproteiineista)
Coronavirus envelope protein: current knowledge
Abstract
Background
Coronaviruses
(CoVs) primarily cause enzootic infections in birds and mammals but, in
the last few decades, have shown to be capable of infecting humans as
well. The outbreak of severe acute respiratory syndrome (SARS) in 2003
and, more recently, Middle-East respiratory syndrome (MERS) has
demonstrated the lethality of CoVs when they cross the species barrier
and infect humans. A renewed interest in coronaviral research has led to
the discovery of several novel human CoVs and since then much progress
has been made in understanding the CoV life cycle.
The CoV envelope (E) protein is a small, integral membrane protein involved in several aspects of the virus’ life cycle, such as assembly, budding, envelope formation, and pathogenesis. Recent studies have expanded on its structural motifs and topology, its functions as an ion-channelling viroporin, and its interactions with both other CoV proteins and host cell proteins.
The CoV envelope (E) protein is a small, integral membrane protein involved in several aspects of the virus’ life cycle, such as assembly, budding, envelope formation, and pathogenesis. Recent studies have expanded on its structural motifs and topology, its functions as an ion-channelling viroporin, and its interactions with both other CoV proteins and host cell proteins.
This
review aims to establish the current knowledge on CoV E by highlighting
the recent progress that has been made and comparing it to previous
knowledge. It also compares E to other viral proteins of a similar
nature to speculate the relevance of these new findings. Good progress
has been made but much still remains unknown and this review has
identified some gaps in the current knowledge and made suggestions for
consideration in future research.
Conclusions
The
most progress has been made on SARS-CoV E, highlighting specific
structural requirements for its functions in the CoV life cycle as well
as mechanisms behind its pathogenesis. Data shows that E is involved in
critical aspects of the viral life cycle and that CoVs lacking E make
promising vaccine candidates. The high mortality rate of certain CoVs,
along with their ease of transmission, underpins the need for more
research into CoV molecular biology which can aid in the production of
effective anti-coronaviral agents for both human CoVs and enzootic CoVs.
Keywords: Coronavirus, Envelope protein, Topology, Assembly, Budding, Viroporin
Background
Coronaviruses (CoVs) (order Nidovirales, family Coronaviridae, subfamily Coronavirinae) are enveloped viruses with a positive sense, single-stranded RNA genome. With genome sizes ranging from 26 to 32 kilobases (kb) in length, CoVs have the largest genomes for RNA viruses. Based on genetic and antigenic criteria, CoVs have been organised into three groups: α-CoVs, β-CoVs, and γ-CoVs (Table 1) [1, 2]. Coronaviruses primarily infect birds and mammals, causing a variety of lethal diseases that particularly impact the farming industry [3, 4]. They can also infect humans and cause disease to varying degrees, from upper respiratory tract infections (URTIs) resembling the common cold, to lower respiratory tract infections (LRTIs) such as bronchitis, pneumonia, and even severe acute respiratory syndrome (SARS) [5–14]. In recent years, it has become increasingly evident that human CoVs (HCoVs) are implicated in both URTIs and LRTIs, validating the importance of coronaviral research as agents of severe respiratory illnesses [7, 9, 15–17].
Alfa.coronaviruses :
Beta coronaviruses ;
Gamma-coronaviruses:
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 [19–23].
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% [24]. 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% [25]. 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 [26–30]. Live, attenuated vaccines and fusion inhibitors have proven promising, but both also require an intimate knowledge of CoV molecular biology [29, 31–36].
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, 39–42]. 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 [42–44].
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 [45–47].
Unlike the other major structural proteins, N is the only protein that functions primarily to bind to the CoV RNA genome, making up the nucleocapsid [48].
Although N is largely involved in processes relating to the viral genome, it is also involved in other aspects of the CoV replication cycle and the host cellular response to viral infection [49]. Interestingly, localisation of N to the endoplasmic reticulum (ER)-Golgi region has proposed a function for it in assembly and budding [50, 51]. However, transient expression of N was shown to substantially increase the production of virus-like particles (VLPs) in some CoVs, suggesting that it might not be required for envelope formation, but for complete virion formation instead [41, 42, 52, 53].
Background
Coronaviruses (CoVs) (order Nidovirales, family Coronaviridae, subfamily Coronavirinae) are enveloped viruses with a positive sense, single-stranded RNA genome. With genome sizes ranging from 26 to 32 kilobases (kb) in length, CoVs have the largest genomes for RNA viruses. Based on genetic and antigenic criteria, CoVs have been organised into three groups: α-CoVs, β-CoVs, and γ-CoVs (Table 1) [1, 2]. Coronaviruses primarily infect birds and mammals, causing a variety of lethal diseases that particularly impact the farming industry [3, 4]. They can also infect humans and cause disease to varying degrees, from upper respiratory tract infections (URTIs) resembling the common cold, to lower respiratory tract infections (LRTIs) such as bronchitis, pneumonia, and even severe acute respiratory syndrome (SARS) [5–14]. In recent years, it has become increasingly evident that human CoVs (HCoVs) are implicated in both URTIs and LRTIs, validating the importance of coronaviral research as agents of severe respiratory illnesses [7, 9, 15–17].
Alfa.coronaviruses :
| α-CoVs | Transmissible gastroenteritis coronavirus (TGEV) |
| Canine coronavirus (CCoV) | |
| Porcine respiratory coronavirus (PRCoV) | |
| Feline coronavirus (FeCoV) | |
| Porcine epidemic diarrhoea coronavirus (PEDV) | |
| Human coronavirus 229E (HCoV-229E) | |
| Human coronavirus NL63 (HCoV-NL63) |
| β-CoVs | Bat coronavirus (BCoV) |
| Porcine hemagglutinating encephalomyelitis virus (HEV) | |
| Murine hepatitis virus (MHV) | |
| Human coronavirus 4408 (HCoV-4408) | |
| Human coronavirus OC43 (HCoV-OC43) | |
| Human coronavirus HKU1 (HCoV-HKU1) | |
| Severe acute respiratory syndrome coronavirus (SARS-CoV) | |
| Middle Eastern respiratory syndrome coronavirus (MERS-CoV) |
| γ-CoVs | Avian infectious bronchitis virus (IBV) |
| Turkey coronavirus (TCoV) |
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 [19–23].
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% [24]. 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% [25]. 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 [26–30]. Live, attenuated vaccines and fusion inhibitors have proven promising, but both also require an intimate knowledge of CoV molecular biology [29, 31–36].
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, 39–42]. 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 [42–44].
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 [45–47].
Unlike the other major structural proteins, N is the only protein that functions primarily to bind to the CoV RNA genome, making up the nucleocapsid [48].
Although N is largely involved in processes relating to the viral genome, it is also involved in other aspects of the CoV replication cycle and the host cellular response to viral infection [49]. Interestingly, localisation of N to the endoplasmic reticulum (ER)-Golgi region has proposed a function for it in assembly and budding [50, 51]. However, transient expression of N was shown to substantially increase the production of virus-like particles (VLPs) in some CoVs, suggesting that it might not be required for envelope formation, but for complete virion formation instead [41, 42, 52, 53].
The M protein is the most abundant structural protein and defines the shape of the viral envelope [54].
It is also regarded as the central organiser of CoV assembly,
interacting with all other major coronaviral structural proteins [29].
Homotypic interactions between the M proteins are the major driving
force behind virion envelope formation but, alone, is not sufficient for
virion formation [54–56].
Interaction of S with M is necessary for retention of S in the ER-Golgi
intermediate compartment (ERGIC)/Golgi complex and its incorporation
into new virions, but dispensable for the assembly process [37, 45, 57].
Binding of M to N stabilises the nucleocapsid (N protein-RNA complex),
as well as the internal core of virions, and, ultimately, promotes
completion of viral assembly [45, 58, 59]. Together, M and E make up the viral envelope and their interaction is sufficient for the production and release of VLPs [37, 60–64].
The
E protein is the smallest of the major structural proteins, but also
the most enigmatic. During the replication cycle, E is abundantly
expressed inside the infected cell, but only a small portion is
incorporated into the virion envelope [65].
The majority of the protein is localised at the site of intracellular
trafficking, viz. the ER, Golgi, and ERGIC, where it participates in CoV
assembly and budding [66].
Recombinant CoVs have lacking E exhibit significantly reduced viral
titres, crippled viral maturation, or yield propagation incompetent
progeny, demonstrating the importance of E in virus production and
maturation [35, 39, 40, 67, 68].
( Seuraavassa sitaattipomintoja uusimmasta E envelope virusproteiinia käsittelevästä a rtikkelsita)
The envelope protein
Structure
The CoV E protein is a short, integral membrane protein of 76–109 amino acids, ranging from 8.4 to 12 kDa in size [69–71].
The primary and secondary structure reveals that E has a short,
hydrophilic amino terminus consisting of 7–12 amino acids, followed by a
large hydrophobic transmembrane domain (TMD) of 25 amino acids, and
ends with a long, hydrophilic carboxyl terminus, which comprises the
majority of the protein (Fig. 1) [1, 60, 72–75].
The hydrophobic region of the TMD contains at least one predicted
amphipathic α-helix that oligomerizes to form an ion-conductive pore in
membranes [76–78].
Amino
Acid Sequence and Domains of the SARS-CoV E Protein. The SARS-CoV E
protein consists of three domains, i.e. the amino (N)-terminal domain,
the transmembrane domain (TMD), and the carboxy (C)-terminal domain.
Amino acid properties are indicated: hydrophobic (red), hydrophilic
(blue), polar, charged (asterisks) [78]
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