Mass determination of Rous sarcoma virus virions by scanning transmission electron microscopy

The internal structural protein of retroviruses, Gag, comprises most of the mass of the virion, and Gag itself can give rise to virus-like particles w hen expressed in appropriate cells. Previously the stoichiometry of Gag in virions was inferred from indirect measurements carried out 2 decades ago. We now have directly determined the masses of individual particles of the p rototypic avian retrovirus, Rous sarcoma virus (RSV), by using scanning tra nsmission electron microscopy. In this technique, the number of scattered e lectrons in the dark-field image integrated over an individual freeze-dried virus particle on a grid is directly proportional to its mass. The RSV vir ions had a mean mass of 2.5 x 10(8) Da, corresponding to about 1,500 Gag mo lecules per virion. The population of virions was not homogeneous, dth abou t one-third to two-thirds of the virions deviating from the mean by more th an 10% of the mass in two respective preparations. The mean masses for viri ons carrying genomes of 7.4 or 9.3 kb were indistinguishable, suggesting th at mass variability is not due to differences in RNA incorporation.