title: Molecular substructure of a viral receptor-recognition protein. The gp17 tail-fiber of bacteriophage T7.
abstract: The bacteriophage T7 tail complex consists of a conical tail-tube surrounded by six kinked tail-fibers, which are oligomers of the viral protein gp17 (Mr 61,400). We have derived a molecular model for the tail-fiber by integrating secondary structure predictions with ultrastructural information obtained by correlation averaging of electron micrographs of negatively stained tail complexes. This model has been further refined by high-resolution scanning transmission electron microscopy of purified fibers, both negatively stained and unstained. Mass measurements made from the latter images establish that the fiber is a trimer of gp17. The proximal half-fiber is a uniform rod, about 2.0 nm in diameter and 16.4 nm long, which we infer to be a triple-stranded coiled-coil, containing three copies of an alpha-helical domain of about 117 residues, starting at Phe151. The distal half-fiber is 15.5 nm long, and is made up of four globules, 3.1 to 4.8 nm in diameter, in rigid linear array: it contains the carboxy-terminal halves (residues approximately 268 to 553) of the constituent gp17 chains, arranged with 3-fold symmetry around its long axis. The amino-terminal domains (residues 1 to 149) link the fiber to the tail-tube. We conclude that the three gp17 chains are quasi-equivalent in the proximal half-fiber, equivalent in the distal half-fiber, and non-equivalent in the kink region that separates the two half-fibers: such localized non-equivalence may represent a general mechanism for the formation of kinked joints in segmented homo-oligomeric proteins.
authors.affiliation: Laboratory of Physical Biology, National Institute of Arthritis, Bethesda, MD 20892.
authors.forename: A C
authors.initials: AC
authors.lastname: Steven
authors.forename: B L
authors.initials: BL
authors.lastname: Trus
authors.forename: J V
authors.initials: JV
authors.lastname: Maizel
authors.forename: M
authors.initials: M
authors.lastname: Unser
authors.forename: D A
authors.initials: DA
authors.lastname: Parry
authors.forename: J S
authors.initials: JS
authors.lastname: Wall
authors.forename: J F
authors.initials: JF
authors.lastname: Hainfeld
authors.forename: F W
authors.initials: FW
authors.lastname: Studier
mesh_terms: D000595:Amino Acid Sequence; D006358:Hot Temperature; D046911:Macromolecular Substances; D008855:Microscopy, Electron, Scanning; D011487:Protein Conformation; D011991:Receptors, Virus; D013604:T-Phages / Q000032:analysis*; D014764:Viral Proteins / Q000302:isolation & purification
chemical_list: D046911:Macromolecular Substances; D011991:Receptors, Virus; D014764:Viral Proteins