Engineering G protein-coupled receptor expression in bacteria

Approximately 800 genes in humans are dedicated to the production of a large superfamily of seven-transmembrane α-helical receptors called G protein-coupled receptors (GPCRs) (1). These proteins are responsible for the intracellular chemical communication and for the sensory input by chemical stimuli (smell and taste). Consistent with their central role in human physiology, GPCRs constitute the largest class of targets for drug discovery with approximately one-third of all approved therapeutics acting by modulating GPCR function (1). Until very recently, however, only one GPCR structure had been solved, that corresponding to the signaling-off state of bovine rhodopsin (2). During the past year, we have experienced a revolution in GPCR structural biology, with five additional structures published, including those of the human β2-adrenergic receptor (β2AR) in the absence and presence of an inverse agonist (3, 4) and the turkey β1AR complexed with a high-affinity antagonist (5). Collectively, these structures have opened new vistas for the structural modeling of other receptors, and for the virtual screening of ligand libraries by in silico ligand docking (6). A new study by Sarkar et al. in this issue of PNAS (7) describes a strategy for engineering GPCRs that may help increase the number of solved structures even further. The structural analysis of GPCRs poses formidable challenges. Most of these proteins are found in very low abundance in their natural tissues (8). With the exception of bovine rhodopsin, which can be isolated from retinae, GPCRs have to be expressed and purified from heterologous hosts. For example, protein expressed in insect cells was used for the crystallization of β1AR and β2AR (3–5). However, heterologous expression typically results in low yields of membrane-integrated receptor, or in protein that is …