FIGURE 8.
Two models for topology and orientation of the trans-SNARE complex in homotypic vacuole fusion. The generally assumed model of a Qabc complex in one fusion partner and an R-SNARE in the other (38, 54) and an alternative model, suggesting that the preferred trans-SNARE topology is QbcR-Qa, i.e. the Qa-SNARE acts as a single subunit on one of the fusion partners (55). The lower panels show the SNARE complexes seen from the juxta-membrane regions. They illustrate how SNARE complexes could exist in a relaxed or tense state. Association of the SNARE complex with dedicated assembly factors, such as SM proteins and Rab effectors, which form part of the HOPS complex, could dictate assembly of the SNARE complex in a twisted orientation and lead to the tense state. Twisting of the coiled-coil domains can augment the mechanical stress they can exert on the membrane via their TMDs.