Viruses are evolved examples of self-assembled structures that are remarkably sophisticated biological machines. Viral capsids and capsid proteins have been used to assemble structures for guided synthesis of inorganic and organic nanostructures, as cages for packaging cargos, and as vectors for gene therapy. The mechanism of packaging even biologically relevant nucleic acids, however, remained unknown.
The Simian Virus 40 (SV40) is a small nonenveloped virus belonging to the polyomavirus family. SV40 viral capsid proteins encapsidate a circular double-stranded (ds) DNA genome of 5,200 base pairs compacted by histone octamers forming a minichromosome structure.
Using small angle x-ray scattering as well as computer modeling, we wish to reveal the solution structure of Simian Virus 40, as well as its response to different solution conditions.
Recombinant VP15 (SV40’s main capsid protein) also assembles cooperatively around different nucleic acids, to form virus-like particles (VLPs) in a process that appears to follow a rapid kinetic mechanism, in which the capsid geometry, depends on the size and structural features of the nucleic acid substrate. Using Time-Resolved Small-Angle X-ray Scattering we have been able to directly visualize, with a time resolution of 50ms, SV40 VP15 encapsidating short ssRNA (524 nucleotides) and long dsDNA (5.2 kbp) molecules. The assembly process around ssRNA yields T = 1 icosahedral particles comprised of 12 pentamers and one RNA molecule. The reaction follows an extremely rapid, two-state kinetic process with no detectable intermediates. The encapsidation reaction around dsDNA yields a T = 7 icosahedral particles similar to the structure of the capsid of SV40. The reaction time scale is of the order of minutes and SAXS indicates the accumulation of intermediates throughout the reaction.
roi.asor@mail.huji.ac.il
