Background: The retroviral structural protein, Gag, is necessary and sufficient for the formation of immature retroviral particles. For Rous Sarcoma Virus (RSV) and Mason Pfiser Monkey (MPMV) the number of Gag proteins per virus particle has been determined and is approximately 1,500 and 1,900–2,100, respectively. Although the number of Gag proteins in an HIV-1 particle has never been determined, it has been assumed that HIV-1 contains roughly 1,500 copies of Gag. We hypothesized that HIV-1 particles contained roughly the same number of Gag proteins as RSV.

Methods: The mass of in vitro and baculovirus produced HIV-1 immature particles was determined by Scanning Transmission Electron Microscopy (STEM) using Tobacco Mosaic virus as a mass standard. The percent of the mass contributed by protein was divided by the molecular weight of Gag to determine the number of Gag proteins per particle.

Results: In vitro produced HIV-1 particles were found to have a mass of approximately 190 MDa, approximately 3 times the mass of equivalent RSV in vitro made particles. This mass translates to over 4,000 copies of Gag protein per virus particle. The mass of the baculovirus produced particles was found to be approximately 500 MDa, which also correlates with over 4,000 copies of Gag per virus particle.

Conclusions: Contrary to our hypothesis, HIV-1 particles contain 2–3 times more Gag proteins than RSV or MPMV particles.