Mass and molecular composition of vesicular stomatitis virus: a scanning transmission electron microscopy analysis

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Abstract
Dark-field scanning transmission electron microscopy was used to perform mass analyses of purified vesicular stomatitis virions, pronase-treated virions, and nucleocapsids, leading to a complete self-consistent account of the molecular composition of vesicular stomatitis virus. The masses obtained were 265.6 +/- 13.3 megadaltons (MDa) for the native virion, 197.5 +/- 8.4 MDa for the pronase-treated virion, and 69.4 +/- 4.9 MDa for the nucleocapsid. The reduction in mass effected by pronase treatment, which corresponds to excision of the external domains (spikes) of G protein, leads to an average of 1,205 molecules of G protein per virion. The nucleocapsid mass, after compensation for the RNA (3.7 MDa) and residual amounts of other proteins, yielded a complement of 1,258 copies of N protein. Calibration of the amounts of M, NS, and L proteins relative to N protein by biochemical quantitation yielded values of 1,826, 466, and 50 molecules, respectively, per virion. Assuming that the remaining virion mass is contributed by lipids in the viral envelope, we obtained a value of 56.1 MDa for its lipid content. In addition, four different electron microscopy procedures were applied to determine the nucleocapsid length, which we conclude to be 3.5 to 3.7 micron. The nucleocapsid comprises a strand of repeating units which have a center-to-center spacing of 3.3 nm as measured along the middle of the strand. We show that these repeating units represent monomers of N protein, each of which is associated with 9 +/- 1 bases of single-stranded RNA. From scanning transmission electron microscopy images of negatively stained nucleocapsids, we inferred that N protein has a wedge-shaped, bilobed structure with dimensions of approximately 9.0 nm (length), approximately 5.0 nm (depth), and approximately 3.3 nm (width, at the midpoint of its long axis). In the coiled configuration of the in situ nucleocapsid, the long axis of N protein is directed radially, and its depth corresponds to the pitch of the nucleocapsid helix.