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Glutamine synthetase forms three- and seven-stranded helical cables.

T G Frey, D Eisenberg, F A Eiserling

Proceedings of the National Academy of Sciences of the United States of America

|September 1, 1975

Summary

Cobaltous ion binding induces glutamine synthetase polymerization into complex cables. Optical diffraction and filtering reveal the three-dimensional structure of these protein cables and their molecular arrangement.

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Area of Science:

Biochemistry
Structural Biology
Protein Polymerization

Background:

Glutamine synthetase (L-glutamate:ammonia ligase) is a crucial enzyme in nitrogen metabolism.
The enzyme exists as two-layered hexagonal molecules.

Purpose of the Study:

To elucidate the three-dimensional structure of glutamine synthetase cables formed upon cobaltous ion binding.
To determine the molecular symmetry and subunit arrangement within these polymerized structures.

Main Methods:

Induction of polymerization using cobaltous ions.
Analysis of electron micrographs using optical diffraction and filtering (Klug and DeRosier procedure).
Three-dimensional reconstruction of the seven-stranded cable structure.

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Main Results:

Cobaltous ion binding causes glutamine synthetase to polymerize into strands, forming three- and seven-stranded cables.
Optical diffraction and filtering enabled interpretable imaging of cable structures.
Reconstruction revealed the D6 symmetry of glutamine synthetase molecules and their oblate ellipsoidal subunit shape.
Detailed arrangement of subunits within molecules and molecules within strands and cables was determined.

Conclusions:

The study successfully reconstructed the 3D structure of glutamine synthetase cables, revealing precise molecular packing.
The findings provide insights into protein quaternary structure and self-assembly mechanisms driven by metal ion binding.