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The size, shape and stability of complement component C9

R G DiScipio1

  • 1Research Institute of Scripps Clinic, Department of Immunology IMM18, La Jolla, CA 92037.

Molecular Immunology
|August 1, 1993
PubMed
Summary
This summary is machine-generated.

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Electron microscopy reveals C9 protein as a globular ellipsoid, contradicting previous scattering data. High concentrations in scattering experiments caused aggregation, leading to overestimated dimensions for monomeric C9.

Area of Science:

  • Biophysics
  • Structural Biology
  • Protein Science

Background:

  • Previous studies using neutron and X-ray scattering suggested a prolate ellipsoid shape for C9.
  • Discrepancies existed between scattering data and expected physical properties of C9.

Purpose of the Study:

  • To determine the accurate three-dimensional structure of monomeric C9.
  • To reconcile conflicting data from electron microscopy and scattering techniques.
  • To investigate the self-assembly mechanism of poly(C9).

Main Methods:

  • Electron microscopy (including high-resolution) of C9 specimens.
  • Determination of hydrodynamic parameters (frictional ratio).
  • Comparison with known protein structures and parameters.

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Main Results:

  • Electron microscopy visualized C9 as a globular ellipsoid (77 x 70 x 52 A).
  • Hydrodynamic analysis yielded a frictional ratio of 1.32, consistent with globular or heart-shaped proteins.
  • Scattering experiments at high concentrations led to protein aggregation, causing overestimated dimensions and radius of gyration.
  • High-resolution electron microscopy revealed poly(C9) as a barrel-stave structure.

Conclusions:

  • Monomeric C9 is a globular ellipsoid, not a prolate ellipsoid as suggested by prior scattering data.
  • Protein aggregation at high concentrations explains the discrepancy in scattering results.
  • Monomeric C9 undergoes significant conformational changes to form the cylindrical poly(C9) structure.