The HOPS/Class C Vps Complex Tethers High‐Curvature Membranes via a Direct Protein–Membrane Interaction

Membrane tethering is a physical association of two membranes before their fusion. Many membrane tethering factors have been identified, but the interactions that mediate inter‐membrane associations remain largely a matter of conjecture. Previously, we reported that the homotypic fusion and protein sorting/Class C vacuolar protein sorting (HOPS/Class C Vps) complex, which has two binding sites for the yeast vacuolar Rab GTPase Ypt7p, can tether two low‐curvature liposomes when both membranes bear Ypt7p. Here, we show that HOPS tethers highly curved liposomes to Ypt7p‐bearing low‐curvature liposomes even when the high‐curvature liposomes are protein‐free. Phosphorylation of the curvature‐sensing amphipathic lipid‐packing sensor (ALPS) motif from the Vps41p HOPS subunit abrogates tethering of high‐curvature liposomes. A HOPS complex without its Vps39p subunit, which contains one of the Ypt7p binding sites in HOPS, lacks tethering activity, though it binds high‐curvature liposomes and Ypt7p‐bearing low‐curvature liposomes. Thus, HOPS tethers highly curved membranes via a direct protein–membrane interaction. Such high‐curvature membranes are found at the sites of vacuole tethering and fusion. There, vacuole membranes bend sharply, generating large areas of vacuole‐vacuole contact. We propose that HOPS localizes via the Vps41p ALPS motif to these high‐curvature regions. There, HOPS binds via Vps39p to Ypt7p in an apposed vacuole membrane.