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How to weigh polymers packaged in a virus capsid


Researchers from the LPS, the Technical University of Wrocław in Poland and the Laue Langevin Institute in Grenoble revealed the remarkable selectivity of viral proteins for polymer packaging and demonstrated that the process is driven by the principles of thermodynamics.

Many viruses are made up of a protein shell called capsid protecting the genome encoded in one or more polynucleotide chains. Genome packaging can be a highly selective albeit elusive self-assembly process. For instance, cowpea chlorotic mottle virus (CCMV) – a nonenveloped icosahedral single-stranded RNA (ssRNA) plant virus – has a multipartite genome consisting of four ssRNA segments distributed in three indistinguishable particles, each of which contains about 3,000 nucleotides. Such a level of selectivity is often ascribed to intricate molecular recognitions. However, for the simplest icosahedral viruses, the packaging problem can be regarded as charged chains interacting with a rigid shell through nonspecific electrostatic interactions. One of the most intriguing results published in the literature revealed an experimental ratio of about –1.6 between the total charge of the genome and the net charge on the capsid interior for a wide range of simple ssRNA viruses. Furthermore, theoretical predictions based on free energy minimization indicated a linear relationship between the total number of packaged monomers and the capsid interior area. To date, no systematic and accurate experimental investigation has been carried out to establish a linear relationship between the amounts of packaged polyelectrolytes and capsid proteins. Apart from the biological interest, a better knowledge of the packaging mechanisms may support the development of functional nanocages for use in nanotechnology and medicine.

 

(Left) Schematic representation of a viral capsid (in gray) containing a negatively charged polymer (in blue). (Right) Transmission electron micrographs of empty and polymer-filled capsids. The capsids are derived from CCMV.

 
The researchers applied the contrast variation method in small-angle neutron scattering on capsids derived from CCMV and containing a deuterated synthetic negatively charged polymer, the poly(styrene sulfonic acid). The experiments were performed at the Institut Laue Langevin, a neutron source delivering one of the highest fluxes worldwide. They enabled to measure separately and with an unprecedented accuracy the mass of packaged polymer as well as that of the capsid, for a wide range of polymer lengths. It turned out that the mass ratio – or equivalently, the charge ratio – is invariant with the polymer length. To do so, capsids either package several chains simultaneously, or selectively retain the shortest chains that could fit the capsid interior. These results demonstrate that the remarkable selectivity of viral proteins for packaging polymers can be essentially explained by thermodynamic principles involving electrostatic interactions. They shed light into the nonspecific origin of the genome selectivity for many viral systems including CCMV.

 

Contact :
Guillaume Tresset

Référence :
Weighing Polyelectrolytes Packaged in Viruslike Particles
G. Tresset, M. Tatou, C. Le Cœur, M. Zeghal, V. Bailleux, A. Lecchi, K. Brach, M. Klekotko, L. Porcar.
Physical Review Letters, 113, 128305, (2014).

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