C2AB exhibits context-dependent mixed antagonist/agonist activity. (A) C2AB was titrated into fusion assays that contained FL syt in Vr. C2AB inhibited FL syt and Ca²⁺-promoted fusion at relatively low concentrations (<1 µM) but promoted fusion at relatively high concentrations (≥1 µM). (B and C) The extent (B) and initial rate (C) of fusion (only the Ca²⁺-dependent component was analyzed) were plotted versus [C2AB] (n = 3). (D) Average C_m traces of WT and syt KO chromaffin cells that did and did not overexpress GFP-C2AB in response to a steplike elevation of [Ca²⁺]_i generated by flash photolysis of caged Ca²⁺ (arrow indicates the flash). (E and F) The amplitudes (the size of the RRP) and rates of the fast release component were plotted for each condition (n ≥ 10). C2AB inhibited release in the presence of endogenous syt but partially rescued fast release in the absence of native syt. fF, femtofarad. All data shown are represented as mean ± SEM.