Guanine-rich nucleic acid sequences challenge the replication, transcription, 
and translation machinery by spontaneously folding into G-quadruplexes, the 
unfolding of which requires forces greater than most polymerases can exert1,2. 
Eukaryotic cells contain numerous helicases that can unfold G-quadruplexes 3 . 
The molecular basis of the recognition and unfolding of G-quadruplexes by 
helicases remains poorly understood. DHX36 (also known as RHAU and G4R1), a 
member of the DEAH/RHA family of helicases, binds both DNA and RNA 
G-quadruplexes with extremely high affinity4-6, is consistently found bound to 
G-quadruplexes in cells7,8, and is a major source of G-quadruplex unfolding 
activity in HeLa cell lysates 6 . DHX36 is a multi-functional helicase that has 
been implicated in G-quadruplex-mediated transcriptional and 
post-transcriptional regulation, and is essential for heart development, 
haematopoiesis, and embryogenesis in mice9-12. Here we report the co-crystal 
structure of bovine DHX36 bound to a DNA with a G-quadruplex and a 3' 
single-stranded DNA segment. We show that the N-terminal DHX36-specific motif 
folds into a DNA-binding-induced α-helix that, together with the OB-fold-like 
subdomain, selectively binds parallel G-quadruplexes. Comparison with unliganded 
and ATP-analogue-bound DHX36 structures, together with single-molecule 
fluorescence resonance energy transfer (FRET) analysis, suggests that 
G-quadruplex binding alone induces rearrangements of the helicase core; by 
pulling on the single-stranded DNA tail, these rearrangements drive G-quadruplex 
unfolding one residue at a time.