Nuthin' but a G (protein) thang: Insights into the Mechanics of G protein Signaling from Sequence and Structure
Lokits, Alyssa Dawn
G protein-coupled receptors (GPCRs) are a large and diverse group of transmembrane receptors which convert extracellular signals into intracellular responses via coupling to heterotrimeric G proteins. In order to integrate diverse extracellular signals into a message the cell can recognize and respond to, conformational changes occur that rewire the interactions between the receptor and heterotrimer in a specific and coordinated manner. By interrogating the structural and sequence-based constraints of these proteins across each of the signaling states, we can infer which residues are necessary for function and selectivity. Two opportunities emerged to construct predictive models for G protein interactions that invite the application of informatics: 1) With advances in genome sequencing, we can reconstruct and reconcile fully resolved phylogenetic histories of G alpha subunit subfamilies; 2) With experimental G protein structures in complex with protein partners, we can model interactions affiliated with signal mechanics. Here 1 and 2 were combined to create quantitative, predictive models of G protein signaling to identify conserved patterns and characteristics necessary for subfamily-specific protein-protein interactions that will ultimately aid in drug discovery. We were able to successfully model and predict a number of residues across the G protein structure acting as the underlying communication network necessary for function. We then turned to evaluate the sequence-based constraints which imping on subfamily-specific function and selectivity. By integrating sequence information, we were able to predict residue motifs necessary for G protein activation and signaling. Key positions from these predictions have been biochemically validated through mutational studies to verify requirements for G protein subfamily-specific interaction with activated GPCRs and to improve the in silico methodologies in an iterative fashion. Overall, our studies have resulted in new understanding of G protein activation, evolution, and function. As GPCRs represent the targets of roughly half of all therapeutics, increasing our understanding of the intracellular transducing element and the system around these proteins is critical for continued improvement and development of therapeutics. As many diseases are the direct cause of erroneous G protein signaling, study of the mechanism of G protein evolution, activation and signaling remains paramount for the improvement of human health.