Cell surface receptor trafficking

RME is a highly specific cellular biologic process by which, as its name implies, various ligands bind to cell surface receptors and are subsequently internalized and trafficked within the cell. In many cells the process of endocytosis is so active that the entire membrane surface is internalized and replaced in less than 0.5 h (19). 

Under some circumstances, lysosomal hydrolases may fail to be properly packaged in the TGN, so they enter the default pathway to the cell surface, where they are secreted. Although these hydrolases do little harm at the nearly neutral pH of most extracellular fluids, they can also be returned to lysosomes by a pathway known as receptor-mediated endocytosis. In this pathway, M6P receptors are sent to the plasma membrane, where they bind escaped lysosomal hydrolases and bring them back to lysosomes through the early and late endosomes. Receptor-mediated endocytosis is a major component of the endocytic pathways for trafficking of membrane proteins and merit more detailed consideration. 

GABAb receptors are heterodimers. Two GABAb receptor subunits have been cloned, R1 and R2. Neither of these appears to express functional receptors on their own, but they are active when coexpressed, suggesting that a dimer is trafficked to the cell surface and forms an active complex. Evidence shows that the R1 subunit contains the GABA binding site while the R2 subunit interacts with the G protein [14]. 

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