The G-quadruplex (G4) resolvase DHX36 efficiently and specifically disrupts DNA G4s via a translocation-based helicase mechanism

摘要

Single-stranded DNA (ssDNA) and RNA regions that include at least four closely spaced runs of three or more consecutive guanosines strongly tend to fold into stable G-quadruplexes (G4s). G4s play key roles as DNA regulatory sites and as kinetic traps that can inhibit biological processes, but how G4s are regulated in cells remains largely unknown. Here, we developed a kinetic framework for G4 disruption by DEAH-box helicase 36 (DHX36), the dominant G4 resolvase in human cells. Using tetramolecular DNA and RNA G4s with four to six G-quartets, we found that DHX36-mediated disruption is highly efficient, with rates that depend on G4 length under saturating conditions (kcat) but not under subsaturating conditions (kcat/Km). These results suggest that a step during G4 disruption limits the kcat value and that DHX36 binding limits kcat/Km. Similar results were obtained for unimolecular DNA G4s. DHX36 activity depended on a 3′ ssDNA extension and was blocked by a polyethylene glycol linker, indicating that DHX36 loads onto the extension and translocates 3′–5′ toward the G4. DHX36 unwound dsDNA poorly compared with G4s of comparable intrinsic lifetime. Interestingly, we observed that DHX36 has striking 3′-extension sequence preferences that differ for G4 disruption and dsDNA unwinding, most likely arising from differences in the rate-limiting step for the two activities. Our results indicate that DHX36 disrupts G4s with a conventional helicase mechanism that is tuned for great efficiency and specificity for G4s. The dependence of DHX36 on the 3′-extension sequence suggests that the extent of formation of genomic G4s may not track directly with G4 stability.