Endocytosis

Interaction with the environment is crucial for the survival of cells, through communication with one another via signaling molecules (e.g. hormones) and nutrient acquisition. One major problem a cell must overcome is how to selectively carry out exchanges with the external environment and still maintain an internal composition that is chemically distinct. The plasma membrane (PM) makes this selectivity possible. While water and small molecules can cross the lipid bilayer by diffusion or transport through the lipid bilayer, larger macromolecules enter a cell by a different mechanism. Endocytosis is the process by which cells internalize extracellular material by forming intracellular membrane-bound vesicles originating from the PM, and subsequently sort the vesicle contents to the appropriate destination. Internalized material includes membrane proteins, PM lipids, and extracellular fluid, and may also include other cells or apoptotic cell fragments, parasites, viruses, nutrients, or activated PM signaling complexes. The critical importance of endocytosis to such a wide variety of physiological processes has made it a focus of intensive investigation by scientists with diverse backgrounds and interests. Indeed, studies of the endocytic pathway are among the most highly interdisciplinary and methodologically innovative in all of modern biology, and the field is currently in a state of exciting and rapid advance. In this chapter, we attempt to summarize our current understanding of the endocytic pathway, focusing on recent progress in elucidating fundamental mechanisms of endocytic membrane trafficking and highlighting selected examples of their diverse physiological functions.