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The hexameric E. coli DnaB helicase can exist in different Quaternary states
X Yu1, M J Jezewska, W Bujalowski
1Department of Cell Biology and Neuroanatomy, University of Minnesota Medical School, Minneapolis 55455, USA.
Journal of Molecular Biology
|May 31, 1996
Summary
The Escherichia coli DnaB helicase exists in two distinct hexameric ring forms (3-fold and 6-fold symmetry), suggesting conformational changes are key to its function.
Area of Science:
- Biochemistry
- Molecular Biology
- Structural Biology
Background:
- The DnaB protein is the essential replicative helicase in Escherichia coli.
- The active form of DnaB is a hexamer, often depicted as a ring with 3-fold symmetry, proposed to be a trimer of dimers.
Purpose of the Study:
- To investigate the structural conformations of the DnaB hexamer under varying nucleotide cofactor conditions.
- To determine the oligomeric state and symmetry of the DnaB ring structures.
Main Methods:
- Utilized scanning transmission electron microscopy (STEM) for mass analysis.
- Examined DnaB protein structures in the presence of different nucleotide cofactors (ATP, ATPγS, AMP-PNP, ADP).
Main Results:
- Identified two distinct DnaB ring forms: one with 3-fold symmetry and another with 6-fold symmetry, irrespective of the nucleotide cofactor.
- Mass analysis confirmed both forms are hexamers, ruling out a trimer-of-dimers model for the 3-fold form.
- Observed intermediate states between the 3-fold and 6-fold symmetric forms.
- Demonstrated that transitions between these hexameric states are cooperative, implying an equilibrium between conformations.
Conclusions:
- The DnaB helicase hexamer exists in at least two distinct conformational states.
- Cooperative transitions between these states suggest a dynamic mechanism for helicase activity.
- The functional significance of these conformational states in DNA replication requires further investigation.