The fusion of yeast vacuoles requires a Rab-GTPase (Ypt7), a tethering complex termed HOPS (homotypic fusion and vacuole protein sorting) and SNAREs. The HOPS complex consists of six subunits and is involved in the initial contact between late endosome (multi vesicular body) and the vacuole. The homologous CORVET complex shares four subunits with the HOPS complex and is required at the endosome. Upon overexpression, I was able to isolate the entire HOPS and stable subcomplexes consisting of two to six subunits. These subcomplexes might represent the core for the assembly, or may be transition intermediates. They could arise when the CORVET complex at the endosome matures into the HOPS complex at the vacuole. Using a structure-function approach, I analysed the HOPS structure via electron microscopy and its function via vacuole fusion assay.