DNA helicases use chemical energy to unwind the two strands of DNA. A subset of these enzymes, belonging to the RecQ family, show a striking preference for complex, multi-stranded DNA structures containing branches and junctions. Such structures are intermediates of DNA repair. Thus, RecQ helicases, present in organisms ranging from bacteria to humans, are thought to regulate repair processes via yet unexplored molecular mechanisms. Their importance is reflected in inherited RecQ helicase deficiencies associated with high cancer predisposition and premature aging in humans.
Figure: DNA geometry-dependent unwinding by RecQ helicase observed in single-molecule magnetic tweezers experiments. Unwinding of DNA structures by RecQ helicase (multi-color domains) results in an increase in the length of the DNA molecule, which is held between the surface and a paramagnetic bead held under constant force. Trajectories show the unwinding process as a function of time. RecQ unwinds hairpin DNA slowly with frequent pauses, whereas it unwinds gapped DNA rapidly and processively (upper black trajectories). The DNA-geometry dependent activity can be mapped to a single domain (the HRDC domain, colored orange) since RecQ constructs lacking the HRDC domain unwind hairpin and gapped DNA with similar kinetics (lower red trajectories).
In their recently published work, the Neuman and Kovács groups assessed the hypothesis that the movement and DNA processing patterns of RecQ helicases depend on the geometry of the DNA substrate. Such a mechanism could enable the continuous sensing of DNA substrate structure during enzymatic action and, in turn, differential processing of DNA metabolic intermediates leading to physiologically beneficial (i.e. precise DNA repair) or harmful outcomes (imprecise DNA repair leading to genetic malfunction and cancer).
Indeed, in their biophysical and biochemical experiments, the HFSP investigators found that RecQ helicase exhibits complex patterns of movement and a preference for certain DNA structural elements. Together these features lead to differential processing of DNA molecules resembling various DNA repair intermediates. Moreover, they mapped this geometry-dependent unwinding activity to a single domain (protein module) unique to RecQ helicases – the Helicase and RNaseD C-terminal (HRDC) domain. Using genetic assays they also showed that, in the absence of these complex molecular behaviors, the “unleashed” helicase activity is detrimental for the precision of DNA-processing cellular reactions. Based on these results, they proposed a model to explain how the “guardian angels” of the genome meet the paradoxical requirements of efficiency and precision, thereby suppressing deleterious events that could lead to cell death or cancer.
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Keir C. Neuman is a senior investigator at the National Heart, Lung, and Blood Institute of the National Institutes of Health in Bethesda, USA. He obtained his Ph.D. in physics from Princeton University in the lab of Steven Block. He carried out post-doctoral research at Stanford University and was an HFSP Long-Term Fellow in the lab of David Bensimon and Vincent Croquette at the Ecole Normale Supérieure in Paris, France. He has contributed to the development of single-molecule approaches, which he employed to elucidate mechanisms of transcriptional pausing, topoisomerase activity and inhibition, and collagen remodeling by matrix metalloproteinases. |
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Mihály Kovács is a professor in the Department of Biochemistry at Eötvös University in Budapest, Hungary. He performed his graduate studies in part at the University of Leicester, UK, and carried out postdoctoral research at the National Heart, Lung, and Blood Institute, Bethesda, USA. He contributed to the elucidation of the motile mechanism of non-muscle myosin II, a key player in cellular shape changes, differentiation, and cytokinesis. He also characterized the mechanism of action of blebbistatin, which has become the most widely used myosin inhibitor. |
The two investigators teamed up for their HFSP-funded project to decipher how DNA helicase enzymes of the RecQ family, known as the “guardian angels” of the genome, restructure DNA molecules in support of precise and efficient DNA repair.
Reference
Shuttling along DNA and directed processing of D-loops by RecQ helicase support quality control of homologous recombination. Harami, G. M., Seol, Y., In, J., Ferencziová, V., Martina, M., Gyimesi, M., Sarlós, K., Kovács, Z., J., Nagy, N. T., Sun, Y., Vellai, T., Neuman, K. C., Kovács, M. (2017): Proc Natl Acad Sci USA 114: E466-E475.