[Endocytosis, intracellular transport and sorting of proteins]

A complicated interaction between outgoing and incoming protein transport occurs in eucaryotic cells. The incoming protein transport commences when protein molecules (ligands) cleave to specific glycoprotein- or glycolipid-receptors on the cell surfaces for endocytosis thereby. This can proceed in two types of membrane vesicle. Endocytosed ligands and receptors are transported from the surface of the cell to its endosomal system where a dissociation of the ligand from its receptor and sorting with regard to its next destination may occur. There are a good many possibilities of intracellular transport, depending on which ligand/receptor complex is at issue. Thus ligands and receptors can be transported back to the cell surface (recycling), across e.g. a polarized epithelial cell (transcytosis) to the lysosomes, and to the cell's Golgi apparatus. Apart from a large number of physiological ligands, viruses and diverse bacterial and plant toxins are also endocytosed. This allows scope for a therapeutic use of endocytosis.