An asymmetric helicase dimer drives 3′-to-5′ DNA unwinding in type II Druantia antiphage defence
Abstract
Druantia antiphage defence systems are widespread in bacteria, yet the type II system has remained functionally uncharacterized. Here we show that DruE, the helicase effector of a type II Druantia system from Pseudomonas protegens Pf-5, is a 3′-to-5′ helicase that preferentially unwinds substrates bearing a 3′ single-stranded overhang at a duplex junction. Seven cryo-EM structures across the ATPase cycle reveal that DruE forms a constitutive asymmetric homodimer in which two sequence-identical protomers adopt distinct roles. One protomer acts as the translocating motor, threading the 3′ tracking strand through a central channel and coupling nucleotide-dependent conformational changes to directional movement. The other protomer acts as a clamp, capturing the displaced 5′ strand and gripping the adjacent duplex to potentially prevent strand reannealing. A structural element in the clamp protomer also serves as a strand-separation wedge, a function analogous to the separation pins of canonical helicases but uniquely provided by the non-translocating subunit. These results establish a mechanochemical framework for type II Druantia-mediated immunity and suggest that DruE targets 3′-ended or fork-like phage replication intermediates to suppress infection.
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