Structural basis for RNA-duplex unwinding by the DEAD-box helicase DbpA

DEAD-box RNA helicases are implicated in most aspects of RNA biology, where these enzymes unwind short RNA duplexes in an ATP-dependent manner. During the central step of the unwinding cycle, the two domains of the helicase core form a distinct closed conformation that destabilizes the RNA duplex, which ultimately leads to duplex melting. Despite the importance of this step for the unwinding process no high resolution structures of this state are available. Here, we employ nuclear magnetic resonance spectroscopy and X-ray crystallography to determine structures of the DEAD- box helicase DbpA in the closed conformation, complexed with substrate duplexes and single- stranded unwinding product. These structures reveal that DbpA initiates duplex unwinding by interacting with up to three base-paired nucleotides and a 5 single-stranded RNA duplex overhang. These high-resolution snapshots, together with biochemical assays, rationalize the destabilization of the RNA duplex and are integrated into a conclusive model of the unwinding process.