Structural dynamics of DNA unwinding by a replicative helicase

Area of Science:

• Molecular Biology
• Biochemistry
• Structural Biology

Background:

• Hexameric helicases are crucial for DNA replication initiation in all life forms.
• Key mechanisms of DNA unwinding, strand separation, and nucleotide hydrolysis coupling remain incompletely understood.

Purpose of the Study:

• To elucidate the mechanism of DNA unwinding by the simian virus 40 large tumour antigen (LTag) helicase.
• To characterize the dynamic relationship between nucleotide hydrolysis and DNA translocation.

Main Methods:

• Cryo-electron microscopy (cryo-EM) to visualize LTag hexamers on DNA.
• Continuous heterogeneity analysis to study conformational dynamics under catalytic conditions.

Main Results:

• LTag forms head-to-head hexamers at replication origins, separating DNA at two sites.
• Helicase translocates DNA via tracking strand pulling and non-tracking strand extrusion.
• ATP hydrolysis acts as an 'entropy switch' to facilitate translocation, not directly power movement.

Conclusions:

• A comprehensive model for replication fork establishment and progression is proposed, applicable to viral and eukaryotic systems.
• Demonstrates entropy-driven allostery as a mechanism for ATP-dependent enzymes to perform mechanical work.
• Provides insights into the coordinated motions enabling efficient DNA unwinding and strand separation.