DNA translocation by the CMG helicase: the helical inchworm model

In all cells, hexameric helicases drive the unwinding of parental chromosomal DNA at replication forks to provide the single-stranded DNA templates required by replicative DNA polymerases. DNA unwinding proceeds via a steric exclusion mechanism in which the helicase encircles and translocates along one DNA strand while sterically excluding the opposite strand from its central channel. The details of how hexameric helicases translocate on single-stranded DNA remain incompletely understood and likely vary among species, as structural and mechanistic features-such as motor domain architecture and translocation polarity-shape helicase function. Recent high-resolution cryo-EM structures of the eukaryotic CMG (Cdc45-MCM-GINS) helicase, including complexes stalled at leading-strand G-quadruplexes, reveal two predominant DNA-bound conformations: planar and spiral. These structures show that different subsets of MCM subunits alternately engage the leading-strand template, defining intermediates of a nonrotary, hand-over-hand translocation mechanism. This mode of translocation differs from the sequential rotary hand-over-hand mechanism proposed for bacterial hexameric helicases, instead resembling that of other ring-shaped ATPase motors and can be described as a variant of the helical inchworm model. The evolution of this mechanism may reflect CMG's specialized role as a replisome organizer, enabling it to coordinate accessory factors and optimize replication fork progression. Together, these findings highlight the mechanistic diversity and evolutionary adaptability of hexameric helicases.