Current applications of mini G proteins to study the structure and function of G protein-coupled receptors

G protein-coupled receptors (GPCRs) regulate intracellular signalling pathways that contribute to virtually all aspects of cell function. Characterising GPCRs in each of their conformational states is key to understanding their mechanism of action, but structure determination of receptors in their active state, bound to a heterotrimeric G protein or beta-arrestin, has proved challenging. A number of G protein surrogates have been developed to simplify this process, including G protein-derived peptides, nanobodies and, most recently, mini G proteins. The aim of these surrogates is to bind the receptor and stabilise its active conformation, whilst eliminating the problems inherent to native signalling proteins, namely their large size, instability and conformational dynamics. Mini G proteins are composed of a single domain from the G protein a-subunit that has been engineered to form a stable complex with GPCRs. They induce comparable pharmacological and structural changes in the receptor to those elicited by heterotrimeric G proteins, and retain their native receptor-coupling specificity. At least one member of each G protein family has been converted into a mini G protein, which means that they can be used to characterise a wide variety of GPCRs. Since their initial publication two years ago, mini G proteins have facilitated the structure determination of three different receptors in their active state and enabled the development of a methodology to thermostabilise GPCRs in their fully active conformation. They have also been used to develop a range of assays that can measure mini G protein coupling to receptors in vitro, and a sensitive cell-based assay that is capable of accurately reporting ligand efficacy and quantifying G protein coupling in vivo. This review presents an overview of the current applications of mini G proteins to study the structure and function of GPCRs.