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Small heat-shock protein structures reveal a continuum from symmetric to variable assemblies
D A Haley1, M P Bova, Q L Huang
1Department of Molecular and Medical Pharmacology and Crump Institute for Biological Imaging, UCLA School of Medicine, Los Angeles, CA 90095, USA.
Journal of Molecular Biology
|April 15, 2000
Summary
Small heat-shock proteins (sHSPs) like Methanococcus jannaschii HSP16.5 exhibit distinct structural symmetries, unlike more variable alpha-crystallin assemblies. Cryo-EM reveals HSP16.5
Area of Science:
- Structural Biology
- Biochemistry
- Molecular Chaperones
Background:
- Small heat-shock proteins (sHSPs) are crucial molecular chaperones preventing protein aggregation.
- sHSPs are found across all organisms and play vital roles in cellular proteostasis.
- Understanding the quaternary structure of sHSPs is key to their chaperone mechanism.
Purpose of the Study:
- To investigate and compare the quaternary structures of various sHSP assemblies using cryo-electron microscopy (cryo-EM).
- To elucidate the structural basis for the chaperone activity of sHSPs.
- To determine the symmetry and variability in sHSP oligomers.
Main Methods:
- Cryo-electron microscopy (cryo-EM) for high-resolution structural analysis.
- Gel-filtration chromatography to assess monodispersity and polydispersity.
- Single particle analysis and reconstruction with and without imposed symmetry.
Main Results:
- Methanococcus jannaschii HSP16.5 forms highly symmetric, monodisperse assemblies with octahedral symmetry.
- Human HSP27 and bovine/human alpha-crystallin assemblies are more polydisperse and irregular.
- Cryo-EM revealed internal density in HSP16.5, likely the N-terminal residues, suggesting they do not sequester substrates in the central cavity.
- Alpha-crystallin reconstructions represent average structures of variable assemblies, with resolution correlating to variability.
Conclusions:
- sHSP assembly and symmetry vary significantly between different sHSP families.
- HSP16.5 exhibits a stable, symmetric structure, contrasting with the dynamic and variable assemblies of alpha-crystallins.
- Cryo-EM provides valuable insights into the quaternary structure and assembly dynamics of sHSPs, impacting our understanding of their chaperone function.