posted on 2021-07-13, 20:06authored byStefan
L. Schaefer, Hendrik Jung, Gerhard Hummer
During infection
the SARS-CoV-2 virus fuses its viral envelope
with cellular membranes of its human host. The viral spike (S) protein
mediates both the initial contact with the host cell and the subsequent
membrane fusion. Proteolytic cleavage of S at the S2′ site
exposes its fusion peptide (FP) as the new N-terminus. By binding
to the host membrane, the FP anchors the virus to the host cell. The
reorganization of S2 between virus and host then pulls the two membranes
together. Here we use molecular dynamics (MD) simulations to study
the two core functions of the SARS-CoV-2 FP: to attach quickly to
cellular membranes and to form an anchor strong enough to withstand
the mechanical force during membrane fusion. In eight 10 μs
long MD simulations of FP in proximity to endosomal and plasma membranes,
we find that FP binds spontaneously to the membranes and that binding
proceeds predominantly by insertion of two short amphipathic helices
into the membrane interface. Connected via a flexible linker, the
two helices can bind the membrane independently, yet binding of one
promotes the binding of the other by tethering it close to the target
membrane. By simulating mechanical pulling forces acting on the C-terminus
of the FP, we then show that the bound FP can bear forces up to 250
pN before detaching from the membrane. This detachment force is more
than 10-fold higher than an estimate of the force required to pull
host and viral membranes together for fusion. We identify a fully
conserved disulfide bridge in the FP as a major factor for the high
mechanical stability of the FP membrane anchor. We conclude, first,
that the sequential binding of two short amphipathic helices allows
the SARS-CoV-2 FP to insert quickly into the target membrane, before
the virion is swept away after shedding the S1 domain connecting it
to the host cell receptor. Second, we conclude that the double attachment
and the conserved disulfide bridge establish the strong anchoring
required for subsequent membrane fusion. Multiple distinct membrane-anchoring
elements ensure high avidity and high mechanical strength of FP–membrane
binding.