THE SYNAPTIC VESICLE CYCLE - A SINGLE VESICLE BUDDING STEP INVOLVING CLATHRIN AND DYNAMIN

Strong evidence implicates clathrin-coated vesicles and endosome-like vacuoles in the reformation of synaptic vesicles after exocytosis, and it is generally assumed that these vacuoles represent a traffic stati on downstream from clathrin-coated vesicles. To gain insight into the mechanisms of synaptic vesicle budding from endosome-like intermediate s, lysed nerve terminals and nerve terminal membrane subfractions were examined by EM after incubations with GTP gamma S. Numerous clathrin- coated budding intermediates that were positive for AP2 and AP180 immu noreactivity and often collared by a dynamin ring were seen. These wer e present not only on the plasma membrane (Takei, K., P.S. McPherson, S.L. Schmid, and P. De Camilli. 1995. Nature (Lend.). 374:186-190), bu t also on internal vacuoles, The lumen of these vacuoles retained extr acellular tracers and was therefore functionally segregated from the e xtracellular medium, although narrow connections between their membran es and the plasmalemma were sometimes visible by serial sectioning. Si milar observations were made in intact cultured hippocampal neurons ex posed to high K+ stimulation, Coated vesicle buds were generally in th e same size range of synaptic vesicles and positive for the synaptic v esicle protein synaptotagmin, Based on these results, we suggest that endosome-like intermediates of nerve terminals originate by bulk uptak e of the plasma membrane and that clathrin- and dynamin-mediated buddi ng takes place in parallel from the plasmalemma and from these interna l membranes. We propose a synaptic vesicle recycling model that involv es a single vesicle budding step mediated by clathrin and dynamin.