Figure 5.

Differential priming by isoforms reveals a second interaction between Munc18-1 and the fusion machinery. A, Release of primed vesicles after rapid Ca²⁺ uncaging in munc18-1 ^−/− cells expressing Munc18-1 or Munc18-2. Untransfected −/− (null) cells were used as control. The top graph shows the increases of intracellular [Ca²⁺] by the flash of UV light. This evokes rapid membrane fusion and concomitant catecholamine release from primed vesicles in the burst phase, followed by replenishment of the primed pool in the sustained phase. Fusion is assayed by membrane capacitance measurements (ΔC _memb) and catecholamines are detected by amperometry (Q _amp). Traces are averages from n cells: n = 12 untransfected; n = 31 Munc18-1 and 30 Munc18-2. B, Mean ± SEM representation of the burst (0–0.5 s) and sustained (0.5–5 s) phases from the membrane capacitance responses. ***p < 0.0001. C, D, Munc18-1 NV was evaluated similarly to Munc18-2 above. n = 5 untransfected cells; n = 36 Munc18-1 NV expressing cells; and n = 31 Munc18-1 expressing cells. **p < 0.001; ***p < 0.0001.