Role of endocytosis in signalling and regulation of G-protein-coupled receptors

Many G-protein-coupled receptors (GPCRs) undergo agonist-induced endocytosi s. Endocytosis contributes to distinct processes that regulate the number a nd functional activity of receptors present in the plasma membrane, contrib uting to the well described processes of receptor sequestration and down-re gulation. Emerging evidence suggests additional functions of endocytosis in mediating GPCR signalling via certain effector pathways, such as mitogen-a ctivated protein kinase modules. The diverse functions of endocytosis raise fundamental questions about the nature of the vesicular carriers and membr ane pathways that mediate the endocytic trafficking of specific GPCRs. Insi ghts into the biochemical and functional properties of endocytic vesicles c ontaining internalized opioid and adrenergic receptors will be discussed. P rogress towards understanding the mechanisms that control the specificity w ith which distinct GPCRs are sorted to specialized subpopulations of endocy tic vesicles will be highlighted.