G-Quadruplex motifs in prion promoter autoregulate its transcription: implication to prion diseases Prashant Pradhan1, Vivekanandan Perumal1, Bishwajit Kundu1, 1Kusuma School of Biological Sciences, Indian Institute of Technology Delhi, Hauz Khas, New Delhi-110016, India
Misfolding of cellular prion protein (PrPC) into an infectious scrapie (PrPSc) conformation forms the pathological hallmark of transmissible spongiform encephalopathies (TSEs). Interaction of G-quadruplex (GQ) motifs with prions is known but a clear implication of this interaction with latter's pathogenicity is not yet understood. We found that the promoter region of the human prion gene (PRNP) incorporates two putative GQ motifs (Q1 and Q2) that could acquire hybrid intra-molecular G-quadruplex structures (Tm>70 oC). Both these GQ motifs displayed high binding affinity (KD ~70-300 nM) towards the natively helical prion structure (PrPC like) but showed very weak or no association with β sheet rich-PrP oligomers (PrPSc like). The relevance of these interactions were further investigated using multiple kinetic and structural probes that include surface plasmon resonance (SPR), fluorescence, NMR, circular dichroism and molecular dynamics (MD) simulations. We deciphered that the N-terminal unstructured region of PrP (residues 23-88) exhibits a resolvase-like activity and is involved in structural unwinding of both the GQ structures. Our cell-based luciferase assays showed that PrPC could auto regulate its expression by binding and resolving the GQs present in its own promoter sequence. The critical role of GQ structural specificity in auto regulatory function of PrP was substantiated by its loss in mutant luciferase constructs. Further, MD simulations indicated that the N-terminal tail of PrP induces increase in solvent exposure, radius of gyration and loss in native hydrogen bonds in both GQ structures. Overall, our results suggest the presence of a feedback transcriptional regulation of PRNP gene by native PrPC through dynamic unwinding of GQs present in its own promoter. Finally, the loss of feedback transcriptional control during PrPSc formation could be a crucial event hallmarking prion pathogenesis.