The role of electrostatic fields in Ras-effector binding and function
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The organization of two or more biological macromolecules into a functioning assembly is critical for many biological functions to occur. This phenomenon is the result of subtle interplay between complimentary structural and electrostatic factors. While a growing protein data bank of solved protein structures provides experimental evidence for studying the structural factors that stabilize protein-protein interface, there has been little advance in experimental determination of the electrostatic contributions. This lack of experimental investigation into protein electrostatics results in an inability to describe or predict how protein-protein complexes are arranged and stabilized. This problem is addressed in this dissertation by use of vibrational Stark effect (VSE) spectroscopy in which the spectral transitions of a vibrational probe are directly related to the strength and direction of the electric fields in the vicinity of the probe. The work presented here details an approach using VSE spectroscopy coupled with molecular dynamics simulation (MD) to interpret the role that electrostatics play in organizing the signaling protein Ras' interactions with its downstream effectors Raf and Ral guanosine dissociation simulator (RalGDS). Each chapter describes a specific set of experiments and MD simulations designed to understand the nature of protein-protein interactions. In Chapter 3, changes in the absorption energy of the nitrile probe at nine positions along the Ras-Ral interface were compared to results of a previous study examining this interface with Ral-based probes, and a pattern of low electrostatic field in the core of the interface surrounded by a ring of high electrostatic field around the perimeter of the interface was found. The areas of conserved Stark shifts are used to help describe electrostatic factors that stabilize the Ras-Ral interface. In Chapter 4, VSE is used to describe an electrostatic origin to the binding tilt between complexes formed between Ras and its two effectors Raf and Ral. There are three regions of conserved Stark effect shifts upon docking with WT Ras between the two effectors, indicating that the docked complexes conserve electrostatic fields, resulting in different binding orientation of otherwise structurally similar proteins. Chapter 5 details the use of MD simulation in correlation with VSE data for 18 mutants of the Ras at the oncogenic position 61 site. The combination of experimental and simulations support the hypothesis that position 61 on Ras is used to coordinate an active site water molecule during native guanosine triphosphate (GTP) hydrolysis.