MACROMOLECULAR STRUCTURE

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Virus Structures have been analyzed in the caes of (i) Herpes Simplex Virus where we have shown by computer image processing of electron micrographs that the major capsomer of the icosahedral capsid is a hexamer of a 155kDa protein, in the form of a cylinder 14nm in length, with a central channel that is about 4nm at the outer surface but much narrower on the inner surface. (ii) The tail fibers of bacteriophage T7 have been analyzed at high resolution by correlating secondary structure predictions of the 60kDa gp17 polypeptide with computer-processed electron images of individual fibers. Each fiber is an oligomer (probably a dimer) of parallel chains that are composed of three domains. The amino-terminal domains link the fibers to the tail; the central domains form a coiled-coil Alpha-helical rope to make up the proximal half-fiber, 19nm in length; the carboxy-terminal domains form inter-chain crossed Beta-sheets which form a linear sequence of four nodules in the distal half-fiber, also 19nm long. In addition we have determined the helical structures of the bacterial fimbriae of sertotypes #2 and #6 from, Bordetella pertussis, using paracrystals of purified fimbriae. In both cases, the fimbria is a single-start helix which contains five copies of the fimbrial portein (22kDa) in two helical turns, corresponding to an axial shift of 13nm. The outer diameter of the fimbria is about 6nm. Radial density mapping of intermediate filaments of vimentin and of epidermal keratin shows them both to have backbones of uniform average desneity with a diameter of 9-10nm surrounded by a shell of relatively diffuse perpheral density extending to an outer diameter of approximately 15nm. The backbone appears to be made up of a helical packing of the conservative Alph-helical rod-domains of the IF subunits (311 amino acids) whereas the peripheral density is contributed by the highly variable non-Alpha-helical end-domains which contain one third of the total mass. This model holds that the end-domain confer the individual functional specificities of different types of intermediate filaments.