Mass analysis of bacteriophage T4 proheads and mature heads by scanning transmission electron microscopy and hydrodynamic measurements

Abstract Quantitative mass analysis of bacteriophage T4 pro-heads by scanning transmission electron microscopy (STEM) revealed a mass of 79.5 ± 0.6 MDa, while hydrodynamic measurements yielded a prohead mass of about 80 MDa. This is 25% less than the prohead mass deduced from its polypeptide composition, and this finding implies that the bacteriophage T4 prohead is built of fewer polypeptide copies than previously reported. In contrast, the mass of mature heads measured by STEM, 194 ± 2 MDa, is in agreement with previous mass measurements of DNA and protein content, and it is consistent with the previously determined stoichiometry. This good agreement of average STEM values for proheads and mature heads with corresponding hydrodynamic measurements suggests that STEM allows faithful evaluation of the masses of large supramolecular assemblies (i.e., ⩾200 MDa) such as whole viruses or cellular organelles.