Abstract
HIV-1 capsid assembly is an essential process in the virus infection cycle. Initiation of capsid assembly involves viral proteins, genomic RNA, and the inner leaflet of the plasma membrane, facilitated by a number of cellular factors1. The viral structural protein Gag plays a number of central roles in this process, including association with the membrane, selective binding of genomic RNA, and oligomerization and packaging to ultimately produce an immature budded pro-viral particle2. While there have been intensive studies regarding the early stages of Gag assembly, there is a lack of consensus on the mechanism for nucleation and growth of Gag complexes3-7. Here we show that myristoylated Gag forms a trimer nucleus in a model membrane that can selectively bind a dimeric RNA containing the packaging signal. Subsequent growth of myristoyl-Gag oligomers requires vRNA, and occurs by addition of 1 or 2 Gag monomers at a time from solution. These data support a model where the immature capsid lattice formation occurs by a gradual lattice edge expansion, following a trimeric nucleation event. The dynamic single molecule data that support this model were recorded using mass photometry, involving full length myristoylated protein, RNA, and lipid together. These data are the first to support a lattice edge expansion model of Gag during early stages of assembly in a biological-relevant setting, providing insights to the fundamental models of virus structural protein assembly process.