Abstract

The fusion of cellular membranes comprises several steps; membrane attachment requires priming of SNAREs and tethering factors by Sec18p/NSF (N-ethylmaleimide sensitive factor) and LMA1. This leads to trans-SNARE pairing, i.e. formation of SNARE complexes between apposed membranes. The yeast vacuole system has revealed two subsequent molecular events: trans-complex formation of V-ATPase proteolipid sectors (V₀) and release of LMA1 from the membrane. We have now identified a hetero-oligomeric membrane integral complex of vacuolar transporter chaperone (Vtc) proteins integrating these events. The Vtc complex associates with the R-SNARE Nyv1p and with V₀. Subunits Vtc1p and Vtc4p control the initial steps of fusion. They are required for Sec18p/NSF activity in SNARE priming, membrane binding of LMA1 and V₀trans-complex formation. In contrast, subunit Vtc3p is required for the latest step, LMA1 release, but dispensible for all preceding steps, including V₀trans-complex formation. This suggests that Vtc3p might act close to or at fusion pore opening. We propose that Vtc proteins may couple ATP-dependent NSF activity to a subset of V₀ sectors in order to activate them for V₀trans-complex formation and/or control fusion pore opening.