Subunit exchange among endolysosomal tethering complexes is linked to contact site formation at the vacuole

Abstract

The hexameric HOPS (homotypic fusion and protein sorting) complex is a conserved tethering complex at the lysosome-like vacuole, where it mediates tethering and promotes all fusion events involving this organelle. The Vps39 subunit of this complex also engages in a membrane contact site between the vacuole and the mitochondria, called vCLAMP. Additionally, four subunits of HOPS are also part of the endosomal CORVET tethering complex. Here, we analyzed the partition of HOPS and CORVET subunits between the different complexes by tracing their localization and function. We find that Vps39 has a specific role in vCLAMP formation beyond tethering, and that vCLAMPs and HOPS compete for the same pool of Vps39. In agreement, we find that the CORVET subunit Vps3 can take the position of Vps39 in HOPS. This endogenous pool of a Vps3-hybrid complex is affected by Vps3 or Vps39 levels, suggesting that HOPS and CORVET assembly is dynamic. Our data shed light on how individual subunits of tethering complexes such as Vps39 can participate in other functions, while maintaining the remaining subcomplex available for its function in tethering and fusion.

Structure of the HOPS tethering complex, a lysosomal membrane fusion machinery

“…Overall, our insights provide a novel blueprint to understand HOPS function, dynamics, and regulation both in fusion and in other functions of its subunits ( van der Beek et al, 2019 ; Elbaz-Alon et al, 2014 ; Hönscher et al, 2014 ; González Montoro et al, 2021 ; González Montoro et al, 2018 ; Wong et al, 2020 ), and imply a general role of tethering complexes as a catalytic part of the fusion machinery.…”

Structure of the endosomal CORVET tethering complex

“…As RING finger domains are structural elements of HOPS and CORVET, they may function as quality control elements to monitor the assembly of the complex. This could also explain, why an intermediate complex between HOPS and CORVET exists, which maintains the core part and can swap functional subunits such as Vps41/Vps8 or Vps39/Vps3, while the free subunit may fulfill auxiliary functions 22 , 60 . When Vps8 alone is overproduced, cells accumulate MVBs proximal to the vacuole 27 , suggesting that Vps8 can constitute a minimal tether.…”

Structure of the endosomal CORVET tethering complex

“…As RING finger domains are structural elements of HOPS and CORVET, they may function as quality control elements to monitor the assembly of the complex. This could also explain, why an intermediate complex between HOPS and CORVET exists, which maintains the core part and can swap functional subunits such as Vps41/Vps8 or Vps39/Vps3, while the free subunit may fulfill auxiliary functions (Montoro et al, 2021; Peplowska et al, 2007). When Vps8 alone is overproduced, cells accumulate MVBs proximal to the vacuole (Markgraf et al, 2009), suggesting that Vps8 can constitute a minimal tether.…”