Fig. 1. Proposed mechanism of dependence of virus particle assembly on nucleic acid.

Purified Gag proteins are soluble, but assemble into virus-like particles when NA is added. The fact that short oligonucleotides (~ 30 bases) can support assembly suggests that oligomerization renders the Gag assembly-competent. The available data suggest that when several Gag molecules are in close proximity, the very high local Gag concentration induces a conformational change, leading to the exposure of new interfaces (shown here in the C-terminal portion of the CA domain; blue) for protein–protein interactions, and in turn to virus particle assembly. In HIV-1, a high concentration appears to induce a helical conformation in SP1, a short “spacer” between NC (red) and CA [18]. This model was originally suggested by Ma and Vogt [8]. MA domain: green.