posted on 2023-12-21, 20:33authored byJoao Medeiros-Silva, Aurelio J. Dregni, Mei Hong
Helical structures in proteins include
not only α-helices
but also 310 and π helices. These secondary structures
differ in the registry of the CO···H–N
hydrogen bonds, which are i to i + 4 for α-helices, i to i + 3 for 310 helices, and i to i + 5 for π-helices. The standard NMR observable of
protein secondary structures are chemical shifts, which are, however,
insensitive to the precise type of helices. Here, we introduce a three-dimensional
(3D) 1H-detected experiment that measures and assigns CO–HN cross-peaks to distinguish the different types of hydrogen-bonded
helices. This hCOhNH experiment combines efficient cross-polarization
from CO to HN with 13C, 15N, and 1H chemical shift correlation to detect the relative proximities
of the COi–Hi+jN spin pairs. We demonstrate this experiment on
the membrane-bound transmembrane domain of the SARS-CoV-2 envelope
(E) protein (ETM). We show that the C-terminal five residues of ETM
form a 310-helix, whereas the rest of the transmembrane
domain have COi–Hi+4N hydrogen bonds that are characteristic of α-helices.
This result confirms the recent high-resolution solid-state NMR structure
of the open state of ETM, which was solved in the absence of explicit
hydrogen-bonding restraints. This C-terminal 310 helix
may facilitate proton and calcium conduction across the hydrophobic
gate of the channel. This hCOhNH experiment is generally applicable
and can be used to distinguish not only different types of helices
but also different types of β-strands and other hydrogen-bonded
conformations in proteins.