The ins and outs of G protein-coupled receptor trafficking

All cells are faced with the complexity of converting a myriad of extracellular stimuli to intracellular signals and appropriate biological responses. This process involves a series of highly orchestrated molecular events, many of which occur at the plasma-membrane interface. Endocytosis is one such event, and functions to transport numerous small molecules and protein cargo. For G protein-coupled receptors (GPCRs), endocytosis serves as a mechanism to regulate cell-surface-receptor levels, thereby, contributing to the regulation of hormonal responsiveness. The trafficking of GPCRs through the endocytic pathway involves three main steps: recruitment of receptors to discrete endocytic sites, internalization and intracellular sorting. Many recent studies have advanced our understanding of these steps. Novel mechanistic insight into the recruitment of GPCRs to sites of endocytosis has been gained and the discovery of novel protein-protein interactions, which define more clearly the processes of internalization and endocytic sorting, have been characterized. In addition, a role for ubiquitin in internalization and sorting to the degradative pathway has emerged.