  During the alphavirus lifecycle, a nucleocapsid core buds through the cell
membrane to acquire an outer envelope of lipid membrane and viral
glycoproteins. However, the presence of a nucleocapsid core is not required for
assembly of infectious particles. To determine the role of the nucleocapsid
core, we develop a coarse-grained computational model with which we investigate
budding dynamics as a function of glycoprotein and nucleocapsid interactions,
as well as budding in the absence of a nucleocapsid. We find that there is a
transition between glycoprotein-directed budding and nucleocapsid-directed
budding which occurs above a threshold strength of nucleocapsid interactions.
The simulations predict that glycoprotein-directed budding leads to
significantly increased size polydispersity and particle polymorphism. This
polydispersity can be qualitatively explained by a theoretical model accounting
for the competition between bending energy of the membrane and the glycoprotein
shell. The simulations also show that the geometry of a budding particle leads
to a barrier to subunit diffusion, which can result in a stalled, partially
budded state. We present a phase diagram for this and other morphologies of
budded particles. Comparison of these structures against experiments could
establish bounds on whether budding is directed by glycoprotein or nucleocapsid
interactions. Although our model is motivated by alphaviruses, we discuss
implications of our results for other enveloped viruses.
