RNA Encapsidation by SV40-Derived
Nanoparticles Follows
a Rapid Two-State Mechanism
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Abstract
Remarkably, uniform virus-like particles self-assemble
in a process
that appears to follow a rapid kinetic mechanism. The mechanisms by
which spherical viruses assemble from hundreds of capsid proteins
around nucleic acid, however, are yet unresolved. Using time-resolved
small-angle X-ray scattering (TR-SAXS), we have been able to directly
visualize SV40 VP1 pentamers encapsidating short RNA molecules (500mers).
This assembly process yields <i>T</i> = 1 icosahedral particles comprised of 12 pentamers and one RNA molecule.
The reaction is nearly one-third complete within 35 ms, following
a two-state kinetic process with no detectable intermediates. Theoretical
analysis of kinetics, using a master equation, shows that the assembly
process nucleates at the RNA and continues by a cascade of elongation
reactions in which one VP1 pentamer is added at a time, with a rate
of approximately 10<sup>9</sup> M<sup>โ1</sup> s<sup>โ1</sup>. The reaction is highly robust and faster than the predicted diffusion
limit. The emerging molecular mechanism, which appears to be general
to viruses that assemble around nucleic acids, implicates long-ranged
electrostatic interactions. The model proposes that the growing nucleo-protein
complex acts as an electrostatic antenna that attracts other capsid
subunits for the encapsidation process