Title: Investigation of Ribonuclease HI handle region dynamics using Solution-state nuclear magnetic resonance spectroscopy, Molecular Dynamic simulations and X-ray crystallography
Description: Ribonuclease HI (RNase HI), a ubiquitous, non-sequence-specific endonuclease, cleaves the RNA strand in RNA/DNA hybrids. The enzyme has roles in replication, genome maintenance, and is the C-terminal domain of retroviral multi-domain reverse transcriptase (RT) proteins. Murine Leukemia Virus (MLV) and Human Immunodeficiency Virus (HIV) are two such retroviruses and their RNase HI (RNHI) domains are necessary for viral replication, making it an attractive drug target. RNase HI has a “handle region”, an extended loop with a large cluster of positive residues, that is critical for substrate recognition. MLV-RNHI is active in isolation and contains a handle region, but, HIV-RNHI is inactive in isolation and does not contain a handle region. HIV-RT, however, has a region in its polymerase domain (positive charge cluster and aromatic cluster) that makes contact with the RNHI domain that may be serving as a “pseudo” handle region; additionally, insertion of a handle region into isolated HIVRNHI restores its activity.

Overall, a breadth of information exists on this region’s dynamics, but important gaps remain unfilled; gaps that may potentially lead to creating effective drugs to treat the above-mentioned viruses. Solution-state nuclear magnetic resonance (NMR) spectroscopy combined with Molecular Dynamic (MD) simulations suggest a model in which the extended handle region domain of the mesophilic Escherichia coli RNHI (EcRNHI) populates "open" (substrate-bindingcompetent) and "closed" (substrate-binding incompetent) states, while the thermophilic Thermus thermophilus RNHI (TtRNHI) mainly populates the closed state at 300 K. In addition, an in silico designed mutant Val98Ala (V98A) EcRNHI was predicted to populate primarily the closed state. Understanding the structural features and internal motions that lead RNase HI to adopt these various conformers is of central importance to better understanding RNase HI’s role in retroviral infection. To formulate a comprehensive model on handle region dynamics, an integrative approach of NMR spectroscopy, X-ray crystallography, and MD simulations is employed.

The sensitivity to internal conformational dynamics at multiple time scales of NMR spectroscopy, molecular range and resolution of X-ray crystallography, and structural interpretations of dynamic processes by MD simulations create a synergistic trio capable of tackling this issue. First, the in silico 2-state Kinetic model is validated through NMR observables that correlate with the respective conformers, thus serving as experimental analogs. The NMR parameters also correlate with the Michaelis constants (KM) for RNHI homologs and help to confirm the in silico predictions of V98A EcRNHI. This study shows the important role of the handle region in modulation of substrate recognition. It also illustrates the power of NMR spectroscopy in dissecting the conformational preferences underlying enzyme function. Next, a deeper dive is taken into handle region dynamics, specifically focusing on residue 88 and the impact its identity has on this region. Its sidechain interactions are shown to directly correlate with handle region conformations and helps to amend the originally proposed in silico 2-state Kinetic model.

Lastly, looking at RNHI handle region dynamics through an evolutionary lens opens the door to uncovering novel mutations that have been previously overlooked or not identified. Through a phylogenetic analysis, researchers have reconstructed seven ancestral RNHI mutants and three of them have been expressed here. The sequence identity of these three ancestral mutants range from 60-87% to extant homologs and this is reflected by similar peak positions in their 15N HSQC spectra. Requisite experiments to assign the NMR backbone have been completed.