Membrane Transport from Early to Late Endosomes

It is now well established that many, if not all, membrane proteins internalized into animal cells appear in early endosomes, where they are sorted. Then, many receptors, as well as other membrane proteins and approximately half of the early endosomal content, are rapidly cycled back to the plasma membrane, whereas other membrane proteins and receptors, as well as the rest of the early endosomal content, are transported to late endosomes. There, a second sorting step occurs, with some components being recycled to the trans-Golgi network (TGN) and others packaged in lysosomes and degraded. Many studies have shown that early and late endosomes exhibit different properties, including subcellular location, morphology, physical properties, luminal pH, and protein composition. However, the mechanisms of membrane transport from early to late endosomes have remained unclear. Our approach has been to reconstitute and investigate this process in vitro.