title: The HOPS proteins hVps41 and hVps39 are required for homotypic and heterotypic late endosome fusion.
abstract: The homotypic fusion and protein sorting (HOPS) complex is a multisubunit tethering complex that in yeast regulates membrane fusion events with the vacuole, the yeast lysosome. Mammalian homologs of all HOPS components have been found, but little is known about their function. Here, we studied the role of hVps41 and hVps39, two components of the putative human HOPS complex, in the endo-lysosomal pathway of human cells. By expressing hemagglutinin (HA)-tagged constructs, we show by immunoelectron microscopy (immunoEM) that both hVps41 and hVps39 associate with the limiting membrane of late endosomes as well as lysosomes. Small interference RNA (siRNA)-mediated knockdown of hVps41 or hVps39 resulted in an accumulation of late endosomes, a depletion in the number of lysosomes and a block in the degradation of endocytosed cargo. Lysosomal pH and cathepsin B activity remained unaltered in these conditions. By immunoEM we found that hVps41 or hVps39 knockdown impairs homotypic fusion between late endosomes as well as heterotypic fusion between late endosomes and lysosomes. Thus, our data show that both hVps41 and hVps39 are required for late endosomal-lysosomal fusion events and the delivery of endocytic cargo to lysosomes in human cells.
authors.affiliation: Department of Cell Biology, University Medical Center Utrecht, Heidelberglaan 100, 3584, CX Utrecht, The Netherlands.
authors.forename: Maaike S
authors.initials: MS
authors.lastname: Pols
authors.forename: Corlinda
authors.initials: C
authors.lastname: ten Brink
authors.forename: Prajakta
authors.initials: P
authors.lastname: Gosavi
authors.forename: Viola
authors.initials: V
authors.lastname: Oorschot
authors.forename: Judith
authors.initials: J
authors.lastname: Klumperman
mesh_terms: D000071183:Autophagy-Related Proteins; D002401:Cathepsin B / Q000378:metabolism; D004705:Endocytosis; D011992:Endosomes / Q000378:metabolism*; D006367:HeLa Cells; D006801:Humans; D006863:Hydrogen-Ion Concentration; D007425:Intracellular Membranes / Q000378:metabolism; D047908:Intracellular Signaling Peptides and Proteins / Q000235:genetics / Q000378:metabolism*; D008247:Lysosomes / Q000378:metabolism; D008561:Membrane Fusion / Q000235:genetics*; D059748:Proteolysis; D034741:RNA, Small Interfering; D033921:Vesicular Transport Proteins / Q000235:genetics / Q000378:metabolism*
chemical_list: D000071183:Autophagy-Related Proteins; D047908:Intracellular Signaling Peptides and Proteins; D034741:RNA, Small Interfering; C485791:VPS39 protein, human; C549959:VPS41 protein, human; D033921:Vesicular Transport Proteins; D002401:Cathepsin B