Quantitative fluorescence imaging of the impact of the nucleocapsid domain deletion on the dynamics of HIV-1 Gag assembly

The human immunodeficiency virus 1 (HIV-1) group-specific antigen (Gag) polyprotein, the main structural protein of HIV-1 is sufficient to mimic the late stages of the viral replication cycle. When expressed in cells, Gag binds to cellular RNAs and assembles at the plasma membrane to form virus-like particles (VLPs) with a morphology similar to that of HIV-1 virions. The nucleocapsid (NC) domain of Gag plays a critical role in the selective encapsidation of the viral genome. In its absence, Gag binds to cellular RNAs, leading to the formation of VLPs. This work focuses on the impact of NC deletion on Gag assembly, VLP formation, and intracellular trafficking. By combining several fluorescence-based quantitative microscopy techniques, we show that in the absence of the NC domain, Gag cytoplasmic oligomerization and formation of initial ribonucleoprotein complexes are impacted, resulting in a significant delay in the kinetics of VLP formation. Interestingly, VLPs formed from the Gag-ΔNC mutant display greater diversity in size and shape. Single particle tracking experiments revealed that VLPs formed at the plasma membrane are immobile, while intracellular VLPs are mostly mobile. For the latter, the motions and diffusion coefficients of VLPs formed by the Gag-ΔNC mutant were highly similar to those of VLPs formed with the wild-type Gag, indicating that the NC domain is not implicated in the intracellular trafficking.These findings illustrate how fluorescence-based microscopy techniques can provide quantitative insights into the role of the NC domain in Gag assembly.