No abstract

The early endosome comprises morphologically distinct regions specialised in sorting cargo receptors. A central question is whether receptors move through a predetermined structural pathway, or whether cargo selection contributes to the generation of endosome morphology and membrane flux. Here, we show that depletion of tumour susceptibility gene 101 impairs the selection of epidermal growth factor receptor away from recycling receptors within the limiting membrane of the early endosome. Consequently, epidermal growth factor receptor sorting to internal vesicles of the multivesicular body and cargo recycling to the cell surface or Golgi complex are inhibited. These defects are accompanied by disruption of bulk flow transport to the lysosome and profound structural rearrangement of the early endosome. The pattern of tubular and vacuolar domains is replaced by enlarged vacuoles, many of which are folded into multicisternal structures resembling the 'Class E' compartments that define several Saccharomyces cerevisiae vacuolar protein sorting mutants. The cisternae are interleaved by a fine matrix but lack other surface elaborations, most notably clathrin.

Endosomal sorting complexes required for transport (ESCRTs) regulate several events involving membrane invagination, including multivesicular body (MVB) biogenesis, viral budding, and cytokinesis. In each case, upstream ESCRTs combine with additional factors, such as Bro1 proteins, to recruit ESCRT-III and the ATPase VPS4 in order to drive membrane scission. A clue to understanding how such diverse cellular processes might be controlled independently of each other has been the identification of ESCRT isoforms. Mammalian ESCRT-I comprises TSG101, VPS28, VPS37A-D, and MVB12A/B. These could generate several ESCRT-I complexes, each targeted to a different compartment and able to recruit distinct ESCRT-III proteins. Here we identify a novel ESCRT-I component, ubiquitin-associated protein 1 (UBAP1), which contains a region conserved in MVB12. UBAP1 binds the endosomal Bro1 protein His domain protein tyrosine phosphatase (HDPTP), but not Alix, a Bro1 protein involved in cytokinesis. UBAP1 is required for sorting EGFR to the MVB and for endosomal ubiquitin homeostasis, but not for cytokinesis. UBAP1 is part of a complex that contains a fraction of total cellular TSG101 and that also contains VPS37A but not VPS37C. Hence, the presence of UBAP1, in combination with VPS37A, defines an endosome-specific ESCRT-I complex.