Architecture of a dodecameric bacterial replicative helicase

Area of Science:

• Molecular Biology
• Structural Biology
• Biochemistry

Background:

• Hexameric DnaB helicases are crucial for DNA replication, typically loaded by initiator proteins and helicase loaders like DnaC in E. coli.
• The requirement for helicase loaders is not universal, as demonstrated by Helicobacter pylori DnaB bypassing DnaC in E. coli.

Purpose of the Study:

• To elucidate the structural basis of Helicobacter pylori DnaB helicase function.
• To investigate the mechanism of DnaB loading in the absence of canonical helicase loaders.

Main Methods:

• X-ray crystallography to determine the structure of the Helicobacter pylori DnaB C-terminal domain (HpDnaB-CTD).
• Biophysical characterization and electron microscopy (EM) to analyze the full-length HpDnaB protein.
• Structural modeling by docking HpDnaB-CTD into EM reconstructions.

Main Results:

• The crystal structure of HpDnaB-CTD revealed a unique two-helix insertion (HPI) in the ATPase domain.
• Full-length HpDnaB forms head-to-head double hexamers, structurally analogous to eukaryotic and archaeal helicases.
• HPI-HPI interactions were identified as key to the hexamerization of the C-terminal domains.

Conclusions:

• The double-hexamer architecture of HpDnaB provides a model for loader-independent helicase loading.
• This finding offers insights into alternative strategies for bacterial DNA replication initiation.