👤 Adrian R Ferré-D'Amaré

DEAH-box helicases, which share a structurally conserved ATPase core, function in all facets of eukaryotic gene expression. While most helicases are highly specialized for their substrates, DHX36 (DEAH-box helicase 36) resolves both DNA and RNA G-quadruplexes. To elucidate the molecular basis of this versatility, we have determined cryo-electron microscopy structures of bovine DHX36 bound to a three-tier RNA G-quadruplex and a six-tier DNA G-quadruplex at 2.6 and 3.4 Å resolution, respectively. Kinetic and smFRET characterizations of structure-guided mutants indicate a key role for the RecA2 domain of the helicase core in DNA vs. RNA discrimination. Furthermore, our structures show that a sequence-divergent RecA2 domain surface loop synergizes with a DHX36-specific N-terminal extension to orthogonally recognize features that specify G-quadruplexes over other nucleic acid structures. Our analysis suggests that recognizing their folded substrates by DEAH-box helicases may generally involve ornamentations of their structural cores acting synergistically with specialized peripheral elements. Show less

DHX36 is a DEAH-box helicase that resolves parallel G-quadruplex structures formed in DNA and RNA. The recent co-crystal structure of DHX36 bound G4-DNA revealed an intimate contact, but did not address the role of ATP hydrolysis in G4 resolving activity. Here, we demonstrate that unlike on G4-DNA, DHX36 displays ATP-independent unfolding of G4-RNA followed by ATP-dependent refolding, generating a highly asymmetric pattern of activity. Interestingly, DHX36 refolds G4-RNA in several steps, reflecting the discrete steps in forming the G4 structure. We show that the ATP-dependent activity of DHX36 arises from the RNA tail rather than the G4. Mutations that perturb G4 contact result in quick dissociation of the protein from RNA upon ATP hydrolysis, while mutations that interfere with binding the RNA tail induce dysregulated activity. We propose that the ATP-dependent activity of DHX36 may be useful for dynamically resolving various G4-RNA structures in cells. Show less

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 exert Show less

The unwinding of nucleic acid secondary structures within cells is crucial to maintain genomic integrity and prevent abortive transcription and translation initiation. DHX36, also known as RHAU or G4R1, is a DEAH-box ATP-dependent helicase highly specific for DNA and RNA G-quadruplexes (G4s). A fundamental mechanistic understanding of the interaction between helicases and their G4 substrates is important to elucidate G4 biology and pave the way toward G4-targeted therapies. Here we analyze how the thermodynamic stability of G4 substrates affects binding and unwinding by DHX36. We modulated the stability of the G4 substrates by varying the sequence and the number of G-tetrads and by using small, G4-stabilizing molecules. We found an inverse correlation between the thermodynamic stability of the G4 substrates and rates of unwinding by DHX36. In stark contrast, the ATPase activity of the helicase was largely independent of substrate stability pointing toward a decoupling mechanism akin to what has been observed for many double-stranded DEAD-box RNA helicases. Our study provides the first evidence that DHX36 uses a local, non-processive mechanism to unwind G4 substrates, reminiscent of that of eukaryotic initiation factor 4A (eIF4A) on double-stranded substrates. Show less