The three-dimensional distribution of RNA and protein in the interior of tomato bushy stunt virus: a neutron low-resolution single-crystal diffraction study.

BACKGROUND: The published high-resolution model of the isometric T = 3 plant virus tomato bushy stunt virus (TBSV) shows the packing in three different environments (A, B, C) of the 180 coat protein subunits of the capsid. It does not, however, account for the localization of either the viral RNA or approximately 25% of the amino acids of the protein subunits, although at least the RNA is rigidly linked to the viral capsid. Solution studies have shown that most of the missing protein is located in an inner shell, and that most of the RNA is sandwiched between the two protein shells.
RESULTS: We have determined the organization of TBSV at 16 A resolution, using neutron single-crystal diffraction. Connections between the two protein shells are confined to the 20 three-fold axes of the virion, where three C-type subunits meet. Much more RNA density is located under the 30 C-C dimers than under the 60 A-B dimers, where we could even identify lagoons of solvent.
CONCLUSIONS: Our results emphasize the importance of the amino termini of the 60 C-type protein subunits not only in the RNA-protein interactions but also in the organization of the coat protein, and, probably, in the assembly of the virion. The lack of equivalence between subunits of classes A or B and subunits of class C is even more pronounced in the interior of the virion than in the outer shell, which possesses icosahedral symmetry.