N-terminal mutants of syb-2 reduce pool size and sustained component when expressed in adrenal chromaffin cells. The N-terminal mutants of syb-2 are shown in Fig. 1. (Ai) N-terminal SNARE destabilization shows a reduced secretory response in exocytosis evoked by Ca²⁺ uncaging. (top) Mean ± SEM of intracellular Ca²⁺ concentration after UV-induced Ca²⁺ uncaging at time = 0.5 s. (middle) Mean capacitance increase. (bottom) Mean amperometric current (thick traces; left ordinate) and amperometric charge (thin traces; right ordinate). All cells were from double syb-2/cellubrevin knockout mice; control cells were infected with the WT protein (WT rescue). (Aii) A detailed view of normalized traces in Ai shows that the kinetics of release are unaffected (normalized capacitance, thick traces; and normalized amperometric charge, thin traces). (Bi and Bii) Kinetic analysis reveals significant reduction of pool sizes in LATA and VAVA mutants but no effect on rate of release. Fitting of exponentials to capacitance responses was used to quantify parameters reflecting time constants (Bi) and sizes (Bii) of the RRP and SRP. The sustained component (sust.) was found as the slope of the capacitance trace between 1.5 and 5.5 s. Mean ± SEM is shown (WT rescue, n = 21 cells; LATA, n = 20 cells; and VAVA, n = 29 cells; and *, P < 0.05; **, P < 0.01; and ***, P < 0.001 in t test).