RNA Encapsidation by SV40-derived Nanoparticles Follows a Rapid Two-State Mechanism

J Am Chem Soc. 2012 May 30;134(21):8823-30. doi: 10.1021/ja2110703. Epub 2012 Mar 13.


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 T = 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(9) M(-1) s(-1). 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.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Capsid Proteins / chemistry*
  • Capsid Proteins / metabolism*
  • Kinetics
  • Models, Molecular
  • Nanoparticles*
  • Nucleic Acid Conformation
  • Protein Multimerization
  • Protein Structure, Quaternary
  • RNA / chemistry
  • RNA / metabolism*
  • Scattering, Small Angle
  • Simian virus 40*
  • Static Electricity
  • X-Ray Diffraction


  • Capsid Proteins
  • VP1 protein, polyomavirus
  • RNA