Structural Analysis of SARS-CoV-2 Spike Protein

The instructor provided to the students a brief introduction
to the most important features of the structure of SARS-CoV-2. The
four major structural proteins are displayed: the envelope (E), membrane
(M), nucleocapsid (N), and spike (S) proteins (Figure 1).^{7 (link)}It is highlighted that spike protein (approximately 180–200
kDa) is the surface glycoprotein anchored to the viral membrane that
plays an essential role when the infection process of SARS-CoV-2 takes
place. This protein is a trimer of three identical protomers (Figure 2). Each protomer
contains three segments: a short intracellular tail (IC), a transmembrane
anchor (TM), and a large ectodomain that extends outward from the
virus which is coated with sugar chains to hide the virus from the
immune system^{8 (link)} and comprises S1 and S2
subunits.
Next, the students are invited to study the ectodomain by analyzing
the requested structural features that they must observe manipulating
PyMOL.
Although hundreds of structures of this spike protein
are already
available in the Protein Data Bank, the one with the code 7DWY(9 (link)) has been selected and must be loaded in a PyMOL session.
They are encouraged to distinguish the four different levels of the
protein structures: primary, secondary, tertiary, and quaternary,
changing the representation of the molecule from lines or wireframe
to cartoon.
They must learn how to select individual residues
or different
chains, how to change their colors, how to generate objects, how to
show and hide different parts of the protein, how to measure distances
and angles for bonds, and how to generate surfaces.
They have
to realize that the spike protein is a complex of three
identical chains. A schematic illustration of the spike protein (Figure 3) is given to the
students, and they must recognize every single domain in the ectodomain,
extracting them as different objects and coloring them in the suggested
color.
The S1 subunit has an N-terminal
domain (NTD) and a receptor-binding
domain (RBD) located in the C-terminal domain, which is implied in
recognition and binding to the host cell receptor. S2 consists of
the fusion peptide (FP), two heptad repeats 1 (HR1 and HR2) which
operate the fusion of viral and host membranes, a transmembrane domain
(TM), and a cytoplasmic tail (CT).
When different species of
coronavirus are compared, the S2 subunit
is highly conserved, but the sequence of the S1 subunit varies greatly.
S1 and S2 are connected to the S1/S2 cleavage site in which specific
proteases act. The cleavage transforms the spike protein into a fusion
competent form that suffers several conformational changes and allows
it to anchor to the host membrane leading to the membrane fusion.^{10 (link)}