Ultrastructural and biochemical evidence of the trimeric nature of frog virus 3 (FV3) six-coordinated capsomers.

Image analysis of freeze-etch replicas of cylindrical aberrant forms of FV3 provided evidence for three morphological subunits protruding from the six-coordinated capsomers. Negatively stained capsomers displayed both triangular and hexagonal profiles which suggests that their innermost portion is pseudohexagonal. Images from underfocused micrographs of capsomers are indicative of a central channel. The trimeric nature of the capsomer has been established by electrophoresis in the presence of Triton X-100, which showed that the molecular weight of the nondissociated capsomer is about 140,000 whereas that of the polypeptide itself is 48,000. This trimeric association does not occur via disulfide bonds, and inside the capsomers there are no free amino groups accessible to the usual bifunctional reagents. Thus, the chemical nature of the interpolypeptide bonds inside the trimers is still unknown. We have previously estimated the triangulation number (T) of FV3 to be 147 or 133 (Darcy-Tripier et al., 1984). The present study, using optical diffraction of the facets of FV3, allowed a better determination of the angle of skewness and is in favor of T = 133 (h = 9, k = 4, 18 degrees).

The organization of frog virus 3 as revealed by freeze-etching.

A variety of freeze-fracture techniques has been employed in this study with the aim of dissecting the frog virus 3 virion and obtaining further information about its architecture. The icosahedral capsid has a skew symmetry with a triangulation number of 133 or 147. The capsomers are closely packed with a center-to-center spacing of 72 A. The inner membrane contains transmembrane proteins which appear as intra-membranous particles on both fracture faces. Rod-like structures (about 100 A in diameter) are present in the virus interior suggesting that the DNA-protein complex is highly organized.