Complex-specific subunits of endosomal tethering factors determine the balance between HOPS/CORVET complexes

The endolysosomal tethering complexes HOPS and CORVET play pivotal roles in the homo- and heterotypic fusion of early and late endosomes, respectively, and HOPS also mediates the fusion of lysosomes with incoming vesicles including late endosomes and autophagosomes. These heterohexameric complexes share their four core subunits that assemble with additional two, complex-specific subunits. These features and the similar structure of the complexes allow the formation of hybrid complexes, and the complex specific subunits may compete for binding to the core. In our study, we decided to gain insight into how human HOPS and CORVET complexes form. We found that the overexpression of CORVET-specific Vps8 or Tgfbrap1 decreased the amount of core proteins Vps11 and Vps18 that are assembled with HOPS-specific subunits Vps41 or Vps39, which suggests reduced amount of intracellular HOPS. In line with this, we observed that the level of lipidated LC3 protein was elevated in these cells and the autophagic cargo p62 showed accumulation in cells overexpressing Vps8, suggesting failure in autophagosome-lysosome fusion in line with loss of HOPS function. In contrast, overexpression of HOPS-specific Vps39 or Vps41 did not affect the levels of these autophagic markers. Finally, we found that a hybrid complex containing Vps39 and Vps8 could be formed in HEK293 cells.