Endocytic membrane trafficking involves the cellular internalization and sorting of extracellular molecules, plasma membrane proteins and lipids. Endocytosis is required for a vast number of functions, including nutrient uptake, cell adhesion and migration, receptor signalling, pathogen entry and cell polarity. It is well established that endocytosis regulates receptor-mediated signalling (including signalling from receptor tyrosine kinases and G protein-coupled receptors). Conversely, it is becoming apparent that signalling events regulate the endocytic pathway — illustrating how these molecular networks are intertwined (see the Review by Sorkin and von Zastrow).

Different types of endocytic processes can be distinguished by the size of the vesicle formed, their cargo and the machineries involved. Endocytosis that is mediated by vesicles coated with clathrin is the best characterized pathway, yet the selection of cargo by adaptor proteins is considerably more complex than initially anticipated (see the Review by Traub). In recent years, non-clathrin-mediated pathways, including macropinocytosis, phagocytosis and caveolin-dependent pathways, have gained considerable interest, which in turn has led to the discovery of new endocytic recycling systems (see the Review by Grant and Donaldson). Further down the endocytic pathway, lysosomes are the primary catabolic compartments of eukaryotic cells that degrade extracellular material internalized via endocytosis and intracellular components sequestered by autophagy. The importance of lysosomal trafficking pathways is emphasized by recent findings describing new roles of lysosomal membrane proteins in cellular physiology and an increasing number of diseases that is recognized as a defect in lysosome biogenesis. (see the Review by Saftig and Klumperman).

This special Focus on endocytosis reflects the diversity and complexity of endocytic trafficking processes and their associated machineries, as exemplified by the four Reviews. The Nature Research library contains other relevant articles, including Reviews, on the broad topic of endocytosis.