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]
Post-translational modifications
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]
Expression and localization
Genomic information
Genomic organisation of isolate Wuhan-Hu-1, the earliest sequenced sample of SARS-CoV-2, indicating the location of the E gene
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]
Virions and viral assembly
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]
Viroporin
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 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]
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]