G4 Recognition of the Tetrad Face by the DHX36 Specific Motif
The protein DHX36 is a helicase enzyme that has the capability to bind and unwind RNA G4s with great specificity [57, 58].
Since DHX36 has significant preference for the parallel versus antiparallel loop orientation, the loop sequences were limited to a single thymine residue which promotes formation of the parallel species.
The observed mode of binding explains DHX36's preference for RNA G4s; since RNA G4s form almost exclusively the parallel strand orientation with propeller type loops, they are expected to have exposed tetrad faces.
For example, the 18-amino-acid peptide from DHX36 is only 2% of the size of the full 1008amino-acid protein.
So far, DHX36 is the only G4-binding protein that has been shown to directly interact with the open guanine-tetrad face of the G4 and this satisfactorily explains its preference for RNA G4s, since the faces of antiparallel G4s are typically obscured by the loops.
This would, obviously, directly compete with DHX36 binding but has also been shown to prevent binding of other RGG domain containing G4-binding proteins, making specific targeting and mechanistic elucidation of their effects challenging [18, 43, 47].
Marushchak et al., "The RNA helicase RHAU (DHX36) suppresses expression of the transcription factor PITX1," Nucleic Acids Research, vol.
Okun et al., "The RNA helicase RHAU (DHX36) unwinds a G4-quadruplex in human telomerase RNA and promotes the formation of the P1 helix template boundary," Nucleic Acids Research, vol.