<p>Recent advances in structural biology and biochemistry have identified proteins downstream of G protein-coupled receptors (GPCRs) as promising drug targets. These proteins are highly regulated to ensure proper physiological responses from extracellular stimuli. Dysregulation of these signaling enzymes can have detrimental consequences, including cardiovascular disease and cancer. Understanding how these proteins are regulated from a structural and biochemical standpoint can therefore be exploited to develop new therapeutics.</p> <p>In this work, the molecular mechanism of regulation of two different proteins downstream of GPCRs is investigated. The first protein, Regulator of G Protein Signaling 17 (RGS17), is involved in numerous processes throughout the body, including the development and progression of lung cancer. This work presents the crystal structure of RGS17 bound to Ca²⁺. Ca²⁺ was found to bind to the same site as the predicted Ga binding surface and increases interactions between RGS17 and Ga_o. Therefore, Ca²⁺ positively regulates RGS17, supporting a mechanism in which Ca²⁺ increases the GTPase activating function of the RZ-family of RGS proteins to ultimately downregulate Ca²⁺ signaling.</p> <p>The second protein, phospholipase Ce (PLCe), has been implicated in cardiac hypertrophy through its production of second messengers. This process is regulated by the small GTPase Rap1A. This work presents insight into the molecular mechanism of Rap1A-dependent activation of PLCe, in which four conserved, hydrophobic residues on the surface of the RA2 domain of PLCe play an essential role. Furthermore, small angle X-ray scattering studies show that binding of Rap1A induces conformational changes in PLCe, resulting in a more compact activated complex. This supports a mechanism in which Rap1A is an allosteric activator of PLCe, inducing conformational changes in PLCe that increase lipid hydrolysis by relieving autoinhibitory interactions and/or by promoting interactions with the cell membrane.</p>