Our understanding of G protein-coupled receptors (GPCRs) has been greatly aided by their relative tractability in terms of pharmacological targeting. These receptors are fairly easy to express in cells and their signaling pathways are amenable to high throughput screening (HTS) technologies. GPCRs couple to a trimeric G-protein structure composed of an alpha subunit and a beta/gamma subunit. The alpha subunit dissociates from beta/gamma upon stimulation of the GPCR and the duration of the alpha subunit signaling is determined by its intrinsic GTPase activity. This GTPase activity can be modulated by regulator of G-protein signaling (RGS) proteins. In terms of GPCR signaling the vast majority of attention has been paid to alpha subunits and part of the reason for this is the availability of molecules (e.g. pertussis and cholera toxins) that target those subunits. Despite this, it is well known that beta/gamma subunits are also capable of generating signaling as these little proteins are known to activate phospholipase C (PLC), PI3Kinase (PI3K) and G-protein receptor kinases (GRKs). Additionally, beta/gamma subunits activate G-protein coupled inwardly rectifying potassium channels (GiRKs) and inhibit certain types of voltage-gated calcium channels (VGCaC). While these signaling mechanisms for beta/gamma are well known, we know relatively little about the physiology of these processes in vivo. This is because we do not have tools to probe the function of beta/gamma pharmacologically. At least not until 2006.
Bonacci et al., Differential targeting of Gbeta/gamma-subunit signaling with small molecules, Science (2006) [free at Science], was the first paper to describe small molecules that target beta/gamma subunits. The first step in discovering these small molecules involved describing a modulatory binding site for beta/gamma function. The authors used phage display of beta/gamma subunits to screen for peptides that bound to these subunits. They discovered a small peptide, SIGK, that bound to a “hotspot” in beta/gamma. They then used this hotspot to screen several thousand molecules computationally for potential binding within this region. They came out of this “virtual screen” with a list of 85 compounds that could then be tested for interference with SIGK binding to beta/gamma. Of these 85 compounds, they found 9 with apparent binding affinity between 0.1 and 60 uM. They then focused on these compounds as potential modulators of beta/gamma signaling.