REVIEW Open Access
Coronavirus envelope protein: current knowledge Dewald Schoeman and Burtram C. Fielding*
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.
Main body: 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 Corona- viridae, 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 vi- ruses. 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 de- grees, 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]. 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 estab- lish 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
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* Correspondence: email@example.com Molecular Biology and Virology Research Laboratory, Department of Medical Biosciences, University of the Western Cape, Cape Town, South Africa
Schoeman and Fielding Virology Journal (2019) 16:69 https://doi.org/10.1186/s12985-019-1182-0
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 ef- fective, 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 pro- teins [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, in- fectious virion, suggesting that some structural proteins might be dispensable or that these CoVs might encode additional proteins with overlapping compensatory func- tions [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 be- tween 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, unin- fected cells. This formation of giant, multinucleated cells, or syncytia, has been proposed as a strategy to allow direct spreading of the virus between cells, sub- verting 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 . Al- though 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 . Interestingly, localisation of N to the endoplasmic reticulum (ER)-Golgi region has proposed a function for it in assembly and budding [50, 51]. How- ever, transient expression of N was shown to substan- tially increase the production of virus-like particles (VLPs) in some CoVs, suggesting that it might not be re- quired 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 . It is also regarded as the central organiser of CoV assembly, inter- acting with all other major coronaviral structural pro- teins . Homotypic interactions between the M proteins are the major driving force behind virion enve- lope formation but, alone, is not sufficient for virion for- mation [54–56]. Interaction of S with M is necessary for retention of S in the ER-Golgi intermediate compart- ment (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 nucleocap- sid (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 replica- tion cycle, E is abundantly expressed inside the infected cell, but only a small portion is incorporated into the vir- ion envelope . 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 . 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].
Main text 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
Table 1 Organisation of CoV species (adapted from Jimenez- Guardeño, Nieto-Torres )
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)