The closely related 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 could allow the formation of hybrid complexes,
and the complex specific subunits may compete for binding to the core. Indeed, our
biochemical analyses revealed the overlap of binding sites for HOPS-specific VPS41
and CORVET-specific VPS8 on the shared core subunit VPS18. 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, indicating reduced
amount of assembled HOPS. In line with this, we observed the elevation of both lipidated,
autophagosome-associated LC3 protein and the autophagic cargo p62 in these cells,
suggesting impaired autophagosome-lysosome fusion. In contrast, overexpression of
HOPS-specific VPS39 or VPS41 did not affect the level of assembled CORVET or autophagy.
VPS8 or Tgfbrap1 overexpression also induced Cathepsin D accumulation, suggesting
that HOPS-dependent biosynthetic delivery of lysosomal hydrolases is perturbed, too.
These indicate that CORVET-specific subunit levels fine-tune HOPS assembly and activity
in vivo.