Recent studies indicate that active zones (AZs) - sites of neurotransmitter release - may be assembled from preassembled AZ precursor vesicles inserted into the presynaptic plasma membrane. Here we report that one putative AZ precursor vesicle of CNS synapses - the Piccolo-Bassoon transport vesicle (PTV) - carries a comprehensive set of AZ proteins genetically and functionally coupled to synaptic vesicle exocytosis. Time-lapse imaging reveals that PTVs are highly mobile, consistent with a role in intracellular transport. Quantitative analysis reveals that the Bassoon, Piccolo, and RIM content of individual PTVs is, on average, half of that of individual presynaptic boutons and shows that the synaptic content of these molecules can be quantitatively accounted for by incorporation of integer numbers (typically two to three) of PTVs into presynaptic membranes. These findings suggest that AZs are assembled from unitary amounts of AZ material carried on PTVs.
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