Real-Time In-Cell NMR Reveals the Intracellular Modulation of GTP-Bound Levels of RAS

The small guanosine triphosphatase (GTPase) RAS serves as a molecular switch in signal transduction, and its mutation and aberrant activation are implicated in tumorigenesis. Here, we perform real-time, in-cell nuclear magnetic resonance (NMR) analyses of non-farnesylated RAS to measure time courses of the fraction of the active GTP-bound form (fGTP) within cytosol of live mammalian cells. The observed intracellular fGTP is significantly lower than that measured in vitro for wild-type RAS as well as oncogenic mutants, due to both decrease of the guanosine diphosphate (GDP)-GTP exchange rate (k_ex) and increase of GTP hydrolysis rate (k_hy). In vitro reconstitution experiments show that highly viscous environments promote a reduction of k_ex, whereas the increase of k_hy is stimulated by unidentified cytosolic proteins. This study demonstrates the power of in-cell NMR to directly detect the GTP-bound levels of RAS in mammalian cells, thereby revealing that the k_hy and k_ex of RAS are modulated by various intracellular factors.

Benefits of time-resolved nonuniform sampling in reaction monitoring: The case of aza-Michael addition of benzylamine and acrylamide

Monitoring of chemical reactions is best carried out using methods that sample the test object at a rate greater than the time scale of the processes taking place. The recently proposed time-resolved nonuniform sampling (TR-NUS) method allows the use of two-dimensional (2D) nuclear magnetic resonance (NMR) spectra for this purpose and provides a time resolution equivalent to that achievable using one-dimensional spectra. Herein, we show that TR-NUS acquired data eliminates 2D spectral line disturbances and enables more accurate signal integration and kinetics conclusions. The considerations are exemplified with a seemingly simple aza-Michael reaction of benzylamine and acrylamide. Surprisingly, the product identification is possible only using 2D spectra, although credible monitoring requires TR-NUS.

Quo Vadis Biomolecular NMR Spectroscopy?

In-cell nuclear magnetic resonance (NMR) spectroscopy offers the possibility to study proteins and other biomolecules at atomic resolution directly in cells. As such, it provides compelling means to complement existing tools in cellular structural biology. Given the dominance of electron microscopy (EM)-based methods in current structure determination routines, I share my personal view about the role of biomolecular NMR spectroscopy in the aftermath of the revolution in resolution. Specifically, I focus on spin-off applications that in-cell NMR has helped to develop and how they may provide broader and more generally applicable routes for future NMR investigations. I discuss the use of ‘static’ and time-resolved solution NMR spectroscopy to detect post-translational protein modifications (PTMs) and to investigate structural consequences that occur in their response. I argue that available examples vindicate the need for collective and systematic efforts to determine post-translationally modified protein structures in the future. Furthermore, I explain my reasoning behind a Quinary Structure Assessment (QSA) initiative to interrogate cellular effects on protein dynamics and transient interactions present in physiological environments.

The Casein Kinase 2-Dependent Phosphorylation of NS5A Domain 3 from Hepatitis C Virus Followed by Time-Resolved NMR Spectroscopy

Hepatitis C virus (HCV) chronically affects millions of individuals worldwide. The HCV nonstructural protein 5A (NS5A) plays a critical role in the viral assembly pathway. Domain 3 (D3) of NS5A is an unstructured polypeptide responsible for the interaction with the core particle assembly structure. Casein kinase 2 (CK2) phosphorylates NS5A-D3 at multiple sites that have mostly been predicted and only observed indirectly. In order to identify the CK2-dependent phosphorylation sites, we monitored the reaction between NS5A-D3 and CK2 in vitro by time-resolved NMR. We unambiguously identified four serine residues as substrates of CK2. The apparent rate constant for each site was determined from the reaction curves. Ser408 was quickly phosphorylated, whereas the three other serine residues reacted more slowly. These results provide a starting point from which to elucidate the role of phosphorylation in the mechanisms of viral assembly-and in the modulation of the viral activity-at the molecular level.