The structure of viruses of the papilloma-polyoma type: V. Tubular variants built of pentamers

Abstract

Tumour virus infection is generally accompanied by the production of hollow, tubular particles which appear to be polymorphic forms of the protein shell of the virus. The two classes of tube recognized previously—wide and narrow—have been studied by electron microscopy combined with optical diffraction and filtering. In the filtered image, which provides a picture of one side of a tube at a time, the wide tubes are seen to be composed of hexamers arranged on a hexagonal plane lattice rolled up into a cylindrical tube—a type of variant structure expected on the theory of virus assembly described by Caspar & Klug (1962). The narrow tubes have a quite novel kind of structure, which, although not anticipated, can also be understood in terms of the theory of virus assembly. The narrow tubes are seen in the filtered images to be built of pentamers arranged in a particular kind of pentagonal tessellation which arises as a result of the bonding of pentamers across 2-fold axes of the surface lattice; this is the same kind of bonding as is found in the hexamer tubes and in the normal virus shell. Two closely related categories of pentamer tube with the same local packing but differing in their helical parameters have been found. The arrangement of pentamers in the “zero-start” tubes can be thought of geometrically as consisting of puckered rings of six pentamers stacked helically above each other, whereas in the “one-start” tubes the rings become the successive turns of a continuous helix containing approximately 6.6 pentamers per turn.