  Chen et al. solved the crystal structure of bovine DHX36 bound to a DNA with
a G-quadruplex (G4) and a single-stranded DNA segment. They believed that the
mechanism they proposed may represent a general model for describing how a
G4-unfolding helicase recognizes and unfolds G4 DNA. Their conclusion is
interesting, however, we noticed that their linear DNA substrate (DNAMyc) that
harbors a Myc-promoter-derived G4-forming sequence was directly used without
pre-folding. This raises the question whether the structure they obtained
really reflects DHX36-mediated G4 recognition and unfolding, or just only
represents a DHX36-binding-induced quasi-folded G4 structure. By a combination
of polymerase extension, DMS footprinting, stopped-flow, and smFRET assays, we
obtained clear evidences that do not support their ATP-independent one-base
translocation structural model. We further revealed that the oscillation of
FRET signal they observed should correspond to a repetitive G4 binding, but not
unfolding, by DHX36.
