STRUCTURE OF BACTERIOPHAGE-T4 FIBRITIN-M - A TROUBLESOME PACKING ARRANGEMENT

Fibritin, a 52 kDa product of bacteriophage T4 gene wac, forms 530 Ang strom long fibers, named whiskers, that attach to the phage neck and p erform a helper function during phage assembly. Fibritin is a homotrim er, with its predominant central domain consisting of 12 consecutive a -helical coiled-coil segments linked together by loops. The central do main is flanked by small globular domains at both ends. Fibritin M is a genetically engineered fragment of the wild type and contains 74 ami no-acid residues corresponding to the last coiled-coil segment and the complete carboxy-terminal domain. The crystals of fibritin M belong t o the rare space group P3 with three crystallographically independent trimers in the unit cell. The structure has been established at 1.85 A ngstrom resolution by combining molecular and isomorphous replacement techniques. One of the two heavy-atom derivatives used was gaseous xen on. A substantial fraction of residues in each independent trimer is d isordered to various extents in proportion to the lack of restraints o n the molecules provided by the lattice contacts. Accurate modeling of the solvent present in the crystals was crucial for achieving good ag reement with experimental data.