  Human immunodeficiency virus (HIV) capsid proteins spontaneously assemble
around the genome into a protective protein shell called the capsid, which can
take on a variety of shapes broadly classified as conical, cylindrical and
irregular. The majority of capsids seen in in vivo studies are conical in
shape, while in vitro experiments have shown a preference for cylindrical
capsids. The factors involved in the selection of the unique shape of HIV
capsids are not well understood, and in particular the impact of RNA on the
formation of the capsid is not known. In this work, we study the role of the
genome and its interaction with the capsid protein by modeling the genomic RNA
through a mean-field theory. Our results show that the confinement free energy
for a homopolymeric model genome confined in a conical capsid is lower than
that in a cylindrical capsid, at least when the genome does not interact with
the capsid, which seems to be the case in in vivo experiments. Conversely, the
confinement free energy for the cylinder is lower than for a conical capsid if
the genome is attracted to the capsid proteins as the in vitro experiments.
Understanding the factors that contribute to the formation of conical capsids
may shed light on the infectivity of HIV particles.
