Role of Gaussian Curvature in the Budding of HIV-1 Viruses

In the immature stage of HIV-1 life cycle, the exocytosis of the viron begins when cone-shaped Gag polyproteins assemble into a shell beneath the surface of the infected cell membrane. The detailed mechanism of this process is still insufficiently understood. As the protein assembles, this shell is observed to possess large irregular holes and the budding process pauses at certain phase. In human cells, exocytosis is completed only when the virus hijacks the cell's fission-inducing (ESCRT) machinery. The presence of large irregular holes, implying a very small line-tension for the Gag assembly, precludes the classical Lipowsky mechanism for budding. In this talk, we propose a new mechanism for viron budding based on the role of Gaussian curvature coupling between the lipid membrane and the Gag-protein shell. We show that this term plays a crucial role in the initial stages of budding. During the later stages, however, formation of buds with pronounced neck are inhibited, thus offering an explanation for the observed pausing.