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Molecular substructure of a viral receptor-recognition protein. The gp17 tail-fiber of bacteriophage T7
A C Steven1, B L Trus, J V Maizel
1Laboratory of Physical Biology, National Institute of Arthritis, Bethesda, MD 20892.
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.
Insights
Researchers modeled the bacteriophage T7 tail fiber, revealing it
Area of Science:
- Structural biology
- Virology
- Biophysics
Background:
- Bacteriophage T7 has a complex tail structure essential for infection.
- The tail fibers are key components, mediating host cell recognition and attachment.
- Understanding the molecular architecture of tail fibers provides insights into viral assembly and function.
Purpose of the Study:
- To determine the molecular model of the bacteriophage T7 tail fiber.
- To elucidate the quaternary structure and domain organization of the gp17 protein within the tail fiber.
- To investigate the structural basis for the kinked morphology of the T7 tail fiber.
Main Methods:
- Integration of secondary structure predictions with ultrastructural data from electron microscopy.
- Correlation averaging of negatively stained electron micrographs of tail complexes.
- High-resolution scanning transmission electron microscopy (STEM) of purified tail fibers.
- Mass measurements using STEM to determine the oligomeric state of gp17.
Main Results:
- The bacteriophage T7 tail fiber is a trimer of the viral protein gp17.
- The proximal half-fiber is a 16.4 nm rod, likely a triple-stranded coiled-coil of alpha-helical domains.
- The distal half-fiber consists of four globules arranged linearly, containing carboxy-terminal domains of gp17 with 3-fold symmetry.
- Amino-terminal domains of gp17 link the fiber to the tail-tube.
Conclusions:
- The gp17 chains exhibit quasi-equivalent, equivalent, and non-equivalent arrangements in different regions of the tail fiber.
- Localized non-equivalence in the kink region suggests a general mechanism for forming kinked joints in segmented homo-oligomeric proteins.
- This detailed molecular model advances our understanding of bacteriophage structure-function relationships and protein assembly principles.