The structure of viruses of the papilloma-polyoma type: III. Structure of rabbit papilloma virus with an appendix on the topography of contrast in negative-staining for electron-microscopy

Abstract

Unfractionated preparations of rabbit papilloma virus have been examined in the electron-microscope by the method of negative-staining. As in our previous study on human wart virus ( Klug & Finch, 1965 ), we find that the majority of particle images arise by contrast by the negative stain of surface detail from both sides of the virus particles, but a proportion of predominantly “one-side” images occurs. An analysis of the latter proves that the outer shell of the particle has the symmetry of the T = 7 (laevo) icosahedral surface lattice ( Caspar & Klug, 1962 ) and is composed of 72 morphological units. The lattice type is therefore the same as that of human wart virus but of different hand. The structure is further confirmed by the fact that several distinctive types of images can be analysed as “two-side” images which are explicable only on the basis of the same model. These images are identical in type to the “two-side” images we reported in the case of human wart virus, as is to be expected, since the sense of hand makes no difference to a “two-side” image. In addition to the normal full virus particles, empty shells and abnormal particles previously reported by other workers, we have observed a class of empty spherical particles which are distinctly smaller (apparent diameter 380 A) than the normal empty particles (∼ 500 A), and which have fewer morphological units. We have also drawn up a tentative scheme of classification of the abnormal or variant types of particle: (i) apparently elongated versions of the normal virus particle; (ii) tubular particles of diameter similar to the virus particle; (iii) tubular particles of about 350 A diameter; and (iv) the small particles referred to above. In the Appendix we describe an objective method of determining from which side of the virus particle the prevailing contrast of the negative stain originates in those cases where a dominantly “one-side” image is produced. The results show that in the case of both human and rabbit papilloma virus particles, it is the side nearer the carbon substrate which is dominantly contrasted. This fact has enabled us to determine the absolute sense of the hand which is present in the T = 7 lattice of these viruses. The results also indicate that the virus particles are flattened to a varying extent on the electron-microscope grid, and enable semiquantitative estimates to be made of the degree of flattening.