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G-quadruplexes (GQs) are four-stranded nucleic acid secondary structures formed by guanosine (G)-rich DNA and RNA sequences. It is becoming increasingly clear that cellular processes including gene expression and mRNA translation are regulated by GQs. GQ structures have been extensively characterized, however little attention to date has been paid to their conformational dynamics, despite the fact that many biological GQ sequences populate multiple structures of similar free energies, leading to an ensemble of exchanging conformations. The impact of these dynamics on biological function is currently not well understood. Recently, structural dynamics have been demonstrated to entropically stabilize GQ ensembles, potentially modulating gene expression. Transient, low-populated states in GQ ensembles may additionally regulate nucleic acid interactions and function. This review will underscore the interplay of GQ dynamics and biological function, focusing on several dynamic processes for biological GQs and the characterization of GQ dynamics by nuclear magnetic resonance (NMR) spectroscopy in conjunction with other biophysical techniques. This article is part of a Special Issue entitled: Biophysics in Canada, edited by Lewis Kay, John Baenziger, Albert Berghuis and Peter Tieleman. [ABSTRACT FROM AUTHOR]

Biological macromolecules are highly flexible and continually undergo conformational fluctuations on a broad spectrum of timescales. It has long been recognized that dynamics have an important role in the action of these molecules. However, the relationship between molecular function and motion is extremely challenging to delineate, because the conformational space available to macromolecules is vast and the relevant excursions can be infrequent and short-lived. Recent advances in solution nuclear magnetic resonance (NMR) spectroscopy permit biomolecular dynamics to be observed with unprecedented detail. Applications of these new NMR techniques to the study of fundamental processes such as binding and catalysis have provided new insights into how living systems operate at an atomic level. [Copyright &y& Elsevier]

NMR Carr–Purcell–Meiboom–Gill (CPMG)relaxationdispersion experiments represent a powerful approach for characterizingprotein internal motions and for gaining insight into fundamentalbiological processes such as protein folding, catalysis, and allostery.In most cases, CPMG data are analyzed assuming that the protein exchangesbetween two different conformational states. Systems exchanging amongmore than two states are far more challenging to characterize by CPMGNMR. For example, in the case of three-state exchange in the fasttime scale regime, it is difficult to uniquely connect the parametersextracted from CPMG analyses with the physical parameters of mostinterest, intercoversion rates, populations, and chemical shift differencesfor exchanging states. We have developed a grid search selection procedurethat allows these physical parameters to be uniquely determined fromCPMG data, based on additional information, which in this study comprisesligand-induced chemical shift perturbations. We applied this approachto the PBX homeodomain (PBX-HD), a three-helix protein with a C-terminalextension that folds into a fourth helix upon binding to DNA. We recentlyshowed that the C-terminal extension transiently folds, even in theabsence DNA, in a process that is likely tied to the cooperative bindingof PBX-HD to DNA and other homeodomains. Using the grid search selectionprocedure, we found that PBX-HD undergoes exchange between three differentconformational states, a major form in which the C-terminal extensionis unfolded, the previously identified state in which the C-terminalextension forms a fourth helix, and an additional state in which theC-terminal extension is misfolded. [ABSTRACT FROM AUTHOR]