Structural basis of DnaB ring opening by the ancestral helicase loader DciA

Ring-shaped helicases must be opened and loaded onto DNA to initiate genome replication. In bacteria, most species lack canonical AAA+ helicase loaders and instead rely on DciA, an evolutionarily conserved, ATP-independent factor whose mechanism has remained unclear. Here we define how DciA remodels the bacterial replicative helicase DnaB. A 3.5 Å cryo-EM structure reveals a DnaB6·DciA3 complex in which DciA binds the periphery of the helicase and stabilizes a right-handed open-helical conformation with a single cracked interface. Three DciA molecules dock onto alternate linker-helix/docking-helix modules, while one N-terminal domain (NTD) engages the cracked interface, occupying the pocket vacated by the displaced linker helix and acting as a wedge to promote ring opening. Functional analyses reveal how DciA couples ATPase suppression with efficient helicase loading through coordinated actions of its NTDs and C-terminal domains. Together, these findings define a minimal, ATP-independent mechanism for helicase loading and show how an ancestral loader remodels DnaB to overcome a fundamental topological barrier.