Endocytosed material can be taken up by macropinocytosis, via clathrin-coated pits, caveolae or non-coated vesicles. Receptor-mediated uptake is more specific and efficient, and the class of receptor also determines the subsequent intracellular trafficking route. Material taken up by receptor-mediated endocytosis may be dissociated in the endosome, directed to the lysosome or despatched for transcytosis. Monoubiquitination is an important signal for proteins to be endocytosed.
The default pathway for endocytosed material is via the early endosome to the late endosome and thence to the lysosome. However some receptors, such as the transferrin receptor, are recycled to the plasma membrane. Other receptors may transfer material across a cell (transcytosis) or to other intracellular compartments.
Phagocytosed material is taken up by binding to receptors that direct material to phagosomes that fuse with lysosomes to form phagolysosomes, where it is degraded. The response of the phagocytic cell depends on the nature of the phagocytosed material and on which receptors are ligated.
Enzymes within lysosomes degrade material that has been endocytosed, as well as cellular proteins tagged with polyubiquitin, defunct organelles, polynucleotides and lipids. Deficiencies of lysosomal enzymes result in failure to break down macromolecules, and this leads to lysosomal storage diseases.
Many cells have specialised endosomal compartments, for example the MIIC compartment in antigen-presenting cells, where antigenic peptides are loaded onto MHC class II molecules.