Figure 10.

Three models of vesicle fusion at retinal ribbon-style synapses. (A) In Model 1, there is a finite number of t-SNARE receptor complexes on the plasma membrane that are formed between syntaxin3B and SNAP-25 at the ribbon-style active zone. Vesicles on the synaptic ribbon (e.g., black arrow), in addition to the occasional vesicle from the cytoplasmic reserve (e.g., gray arrow), avail themselves of these fusion sites to form the RP. The RRP is comprised of those vesicles that form SNARE complexes at the ribbon bases, where the Ca²⁺ channels cluster. (B) In Model 2, SNARE complexes are formed between synaptobrevin on synaptic vesicles and the t-SNAREs syntaxin3B and SNAP-25 that are located on the plasma membrane at the ribbon-style active zone. Both the RRP and the RP reside on the plasma membrane. (C) In Model 3, vesicles on the synaptic ribbon form intervesicular SNARE complexes. Vesicles at the ribbon bases additionally form SNARE complexes with plasma membrane t-SNAREs. Vesicles can fuse individually, as shown on the left side of the ribbon, or via compound fusion, as shown on the right side of the ribbon. For all, synaptic ribbons, vesicle tethers and plasma membrane are shown in black. Synaptic vesicles (blue) contain synaptobrevin (green). Syntaxin3B (red) and SNAP-25 (dark blue) are found on the plasma membrane (A–C) and in low numbers on the synaptic vesicle membranes, but vesicular syntaxin3B and vesicular SNAP-25 are depicted only when they participate in intervesicular SNARE complexes (C). Fusion-competent vesicles (RP and RRP) are denoted by the presence of a SNARE complex. RRP vesicles are additionally outlined in orange.