A SNARE complex mediating fusion of late endosomes defines conserved properties of SNARE structure and function

Sets of SNARE proteins mediate membrane fusion by assembling into core comp lexes. Multiple SNAREs are thought to function in different intracellular t rafficking steps but it is often unclear which of the SNAREs cooperate in i ndividual fusion reactions. We report that syntaxin 7, syntasin 8, vti1b an d endobrevin/ VAMP-8 form a complex that functions in the fusion of late en dosomes. Antibodies specific for each protein coprecipitate the complex, in hibit homotypic fusion of late endosomes in vitro and retard delivery of en docytosed epidermal growth factor to lysosomes. The purified proteins form core complexes with biochemical and biophysical properties remarkably simil ar to the neuronal core complex, although each of the four proteins carries a transmembrane domain and three have independently folded N-terminal doma ins. Substitution experiments, sequence and structural comparisons revealed that each protein occupies a unique position in the complex, with syntaxin 7 corresponding to syntasin 1, and vti1b and syntaxin 8 corresponding to t he N- and C-terminal domains of SNAP-25, respectively. We conclude that the structure of core complexes and their molecular mechanism in membrane fusi on is highly conserved between distant SNAREs.