Late endosomes: Sorting and partitioning in multivesicular bodies

Late endosomes, which have the morphological characteristics of multivesicu lar bodies, have received relatively little attention in comparison with ea rly endosomes and lysosomes. Recent work in mammalian and yeast cells has g iven insights into their structure and function, including the generation o f their multivesicular morphology. Lipid partitioning to create microdomain s enriched in specific lipids is observed in late endosomes, with some lume nal vesicles enriched in lysobisphosphatidic acid and others in phosphatidy linositol 3-phosphate. Sorting of membrane proteins into the lumenal vesicl es may occur because of the properties of their trans-membrane domains, or as a result of tagging with ubiquitin. Yeast class E Vps proteins and their mammalian orthologs are the best candidates to make up the protein machine ry that controls inward budding, a process that starts in early endosomes. Late endosomes are able to undergo homotypic fusion events and also heterot ypic fusion with lysosomes, a process that delivers endocytosed macromolecu les for proteolytic degradation.