Transmembrane conformation of the envelope protein of an alpha coronavirus, NL63.

The envelope (E) proteins of coronaviruses (CoVs) form cation-conducting channels that are associated with the pathogenicity of these viruses. To date, high-resolution structural information about these viroporins is limited to the SARS-CoV E protein. To broaden our structural knowledge of other members of this family of viroporins, we now investigate the conformation of the E protein of the human coronavirus (hCoV), NL63. Using two- and three-dimensional magic-angle-spinning NMR, we have measured C and N chemical shifts of the transmembrane domain of E (ETM), which yielded backbone (ϕ, ψ) torsion angles. We further measured the water accessibility of NL63 ETM at neutral pH versus acidic pH in the presence of Ca ions. These data show that NL63 ETM adopts a regular α-helical conformation that is unaffected by pH and the N-terminal ectodomain. Interestingly, the water accessibility of NL63 ETM increases only modestly at acidic pH in the presence of Ca compared to neutral pH, in contrast to SARS ETM, which becomes much more hydrated at acidic pH. This difference suggests a structural basis for the weaker channel conductance of α-CoV compared to β-CoV E proteins. The weaker E channel activity may in turn contribute to the reduced virulence of hCoV-NL63 compared to SARS-CoV viruses.