The envelope (E) protein is the smallest and least well-characterized of the four major structural proteins found in coronavirus virions.[2][3][4] It is an integral membrane protein less than 110 amino acid residues long;[2] in SARS-CoV-2, the causative agent of Covid-19, the E protein is 75 residues long.[5] Although it is not necessarily essential for viral replication, absence of the E protein may produce abnormally assembled viral capsids or reduced replication.[2][3] E is a multifunctional protein[6] and, in addition to its role as a structural protein in the viral capsid, it is thought to be involved in viral assembly, likely functions as a viroporin, and is involved in viral pathogenesis.[2][5]
Envelope protein | |||||||||
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Identifiers | |||||||||
Symbol | CoV_E | ||||||||
Pfam | PF02723 | ||||||||
InterPro | IPR003873 | ||||||||
PROSITE | PS51926 | ||||||||
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The E protein consists of a short hydrophilic N-terminal region, a hydrophobic helical transmembrane domain, and a somewhat hydrophilic C-terminal region. In SARS-CoV and SARS-CoV-2, the C-terminal region contains a PDZ-binding motif (PBM).[2][5] This feature appears to be conserved only in the alpha and beta coronavirus groups, but not gamma.[2] In the beta and gamma groups, a conserved proline residue is found in the C-terminal region likely involved in targeting the protein to the Golgi.[2]
The transmembrane helices of the E proteins of SARS-CoV and SARS-CoV-2 can oligomerize and have been shown in vitro to form pentameric structures with central pores that serve as cation-selective ion channels.[5] Both viruses' E protein pentamers have been structurally characterized by nuclear magnetic resonance spectroscopy.[5][7]
The membrane topology of the E protein has been studied in a number of coronaviruses with inconsistent results; the protein's orientation in the membrane may be variable.[3] The balance of evidence suggests the most common orientation has the C-terminus oriented toward the cytoplasm.[8] Studies of SARS-CoV-2 E protein are consistent with this orientation.[5][9]
In some, but not all, coronaviruses, the E protein is post-translationally modified by palmitoylation on conserved cysteine residues.[2][8] In the SARS-CoV E protein, one glycosylation site has been observed, which may influence membrane topology;[8] however, the functional significance of E glycosylation is unclear.[2] Ubiquitination of SARS-CoV E has also been described, though its functional significance is also not known.[2]
NCBI genome ID | 86693 |
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Genome size | 29,903 bases |
Year of completion | 2020 |
Genome browser (UCSC) |
The E protein is expressed at high abundance in infected cells. However, only a small amount of the total E protein produced is found in assembled virions.[2][4] E protein is localized to the endoplasmic reticulum, Golgi apparatus, and endoplasmic-reticulum–Golgi intermediate compartment (ERGIC), the intracellular compartment that gives rise to the coronavirus viral envelope.[2][5]
Studies in different coronaviruses have reached different conclusions about whether E is essential to viral replication. In some coronaviruses, including MERS-CoV, E has been reported to be essential.[10] In others, including mouse coronavirus[11] and SARS-CoV, E is not essential, though its absence reduces viral titer,[12] in some cases by introducing propagation defects or causing abnormal capsid morphology.[2]
The E protein is found in assembled virions where it forms protein-protein interactions with the coronavirus membrane protein (M), the most abundant of the four structural proteins contained in the viral capsid.[2][4] The interaction between E and M occurs through their respective C-termini on the cytoplasmic side of the membrane.[2] In most coronaviruses, E and M are sufficient to form virus-like particles,[2][4] though SARS-CoV has been reported to depend on N as well.[14] There is good evidence that E is involved in inducing membrane curvature to create the typical spherical coronavirus virion.[2][15] It is likely that E is involved in viral budding or scission, although its role in this process has not been well characterized.[2][4][15]
In its pentameric state, E forms cation-selective ion channels and likely functions as a viroporin.[5] NMR studies show that viroporin presents an open conformation at low pH or in the presence of calcium ions, while the closed conformation is favored at basic pH.[16] The NMR structure shows a hydrophobic gate at leucine 28 in the middle of the pore. The passage of ions through the gate is thought to be facilitated by the polar residues at the C-terminus.[17]
The cation leakage may disrupt ion homeostasis, alter membrane permeability, and modulate pH in the host cell, which may facilitate viral release.[2][4]
The E protein's role as a viroporin appears to be involved in pathogenesis and may be related to activation of the inflammasome.[3][18] In SARS-CoV, mutations that disrupt E's ion channel function result in attenuated pathogenesis in animal models despite little effect on viral growth.[10]
Protein-protein interactions between E and proteins in the host cell are best described in SARS-CoV and occur via the C-terminal PDZ domain binding motif. The SARS-CoV E protein has been reported to interact with five host cell proteins: Bcl-xL, PALS1, syntenin, sodium/potassium (Na+/K+) ATPase α-1 subunit, and stomatin.[2] The interaction with PALS1 may be related to pathogenesis via the resulting disruption in tight junctions.[3][10] This interaction has also been identified in SARS-CoV-2.[19]
The sequence of the E protein is not well conserved across coronavirus genera, with sequence identities reaching under 30%.[12] In laboratory experiments on mouse hepatitis virus, substitution of E proteins from different coronaviruses, even from different groups, could produce viable viruses, suggesting that significant sequence diversity can be tolerated in functional E proteins.[20] The SARS-CoV-2 E protein is very similar to that of SARS-CoV, with three substitutions and one deletion.[4] A study of SARS-CoV-2 sequences suggests that the E protein is evolving relatively slowly compared to other structural proteins.[21] The conserved nature of the envelope protein among SARS-CoV and SARS-CoV-2 variants has led it to be researched as a potential target for universal coronavirus vaccine development.[22][23]