Protein sorting during endocytosis

Abstract

Movement and sorting of proteins along the endocytic pathway is complex. At most branching points in the pathway movement can potentially be multidirectional. For example, a protein passing through the endosomal membrane system has access to transport routes that could take it ‘back’ to the cell surface, ‘forward’ to the lysosomes, or in a ‘lateral’ direction towards the trans Golgi network (TGN). Secondly, movement along the endocytic pathway involves multiple terminal destinations, most notably the cell surface (either apical or basolateral in polarized cells), the lysosomes, or the TGN. Thirdly, in some instances multiple pathways can be used to achieve the same final destination. This is most evident in the case of some lysosomal proteins where targeting to lysosomes can be either intracellular (directly from the TGN) or via the cell surface following internalization.