In the assay for the homotypic fusion of yeast vacuolar vesicles it was necessary to incubate the vesicles from two strains with NSF and ATP before they were mixed together. If they were not pre-treated in this way, the vesicles would not fuse. Why do you suppose that step was necessary? Would you expect that such treatment would be required for vesicles that each carried just one kind of SNARE ( a t-SNARE or a v-SNARE)?