DNA translocation by the CMG helicase: the helical inchworm model

Area of Science:

• Molecular Biology
• Biochemistry
• Structural Biology

Background:

• Hexameric helicases are essential for DNA replication, unwinding parental DNA at replication forks.
• DNA unwinding occurs via steric exclusion, where helicases translocate along one strand while excluding the other.
• The precise translocation mechanism of hexameric helicases on single-stranded DNA is not fully understood and varies across species.

Purpose of the Study:

• To elucidate the translocation mechanism of the eukaryotic CMG (Cdc45-MCM-GINS) helicase.
• To investigate the structural basis of CMG helicase function at replication forks, particularly when encountering G-quadruplex structures.

Main Methods:

• High-resolution cryo-electron microscopy (cryo-EM) was used to determine the structures of the CMG helicase.
• Complexes of CMG helicase stalled at leading-strand G-quadruplexes were analyzed to capture functional intermediates.

Main Results:

• Two predominant DNA-bound conformations, planar and spiral, were identified for the CMG helicase.
• The eukaryotic CMG helicase utilizes a nonrotary, hand-over-hand translocation mechanism, distinct from bacterial helicases.
• This mechanism involves alternating engagement of MCM subunits with the leading-strand DNA template.

Conclusions:

• The CMG helicase employs a unique translocation mechanism, resembling a helical inchworm model, which differs from rotary mechanisms in bacteria.
• This specialized mechanism may facilitate CMG's role as a replisome organizer, coordinating factors for efficient replication fork progression.
• Findings reveal the mechanistic diversity and evolutionary adaptability of hexameric helicases in different cellular contexts.