  HIV-1, an enveloped RNA virus, produces viral particles that are known to be
much more heterogeneous in size than is typical of non-enveloped viruses. We
present here a novel strategy to study HIV-1 Viral Like Particles (VLP)
assembly by measuring the size distribution of these purified VLPs and
subsequent viral cores thanks to Atomic Force Microscopy imaging and
statistical analysis. This strategy allowed us to identify whether the presence
of viral RNA acts as a modulator for VLPs and cores size heterogeneity in a
large population of particles. These results are analyzed in the light of a
recently proposed statistical physics model for the self-assembly process. In
particular, our results reveal that the modulation of size distribution by the
presence of viral RNA is qualitatively reproduced, suggesting therefore an
entropic origin for the modulation of RNA uptake by the nascent VLP.
