Common Aspects of G-Quadruplex Destabilization among Helicases and Single Stranded DNA Binding Proteins

Abstract

Various helicases and single-stranded DNA (ssDNA) binding proteins are known to destabilize G-quadruplex (GQ) structures, which otherwise result in genomic instability. Bulk biochemical studies have shown that Bloom helicase (BLM) unfolds both intermolecular and intramolecular GQ in the presence of ATP. Using single molecule FRET, we show that binding of BLM to ssDNA in the vicinity of an intramolecular GQ leads to unfolding of the GQ in the absence of ATP. We show that the efficiency of BLM-mediated GQ unfolding correlates with the binding stability of BLM to ssDNA overhang, as modulated by the nucleotide state, ionic conditions, overhang length, and overhang directionality. A related protein WRN showed similar activity to BLM. These results are surprisingly similar to those we observed on interactions of ssDNA binding protein Replication Protein A with GQ, which also does not require ATP or enzymatic activity to unfold GQ. These similarities point out to common features of GQ destabilization mechanisms of helicases and ssDNA binding proteins, in which binding of the protein is what initiates and in some instances is adequate to unfold the GQ.