Fig. 2. Fab fragments specific for endobrevin, vti1b, syntaxin 7 and syntaxin 8 inhibit homotypic fusion of late endosomes. Homotypic fusion of early and late endosomes derived from PC12 cells was monitored in vitro using a content-mixing assay. Fab fragments (1.2 µM) specific for cellubrevin, endobrevin, vti1a, vti1b, syntaxin 7 and syntaxin 8 were used for pre-incubation of post-nuclear supernatants. ATP-dependent fusion activity in the absence of Fab fragments is defined as 100%. Values are given as means of three independent experiments, bars indicate the range. Fusion was initiated by mixing donor and acceptor fractions while simultaneously adding ATP and cytosol, followed by 30 min of incubation. See legend to Figure 1 for abbreviations.