Fig. 7. Kinetics of Ca²⁺-triggered CV–CV fusion. (A) Differential effects on Ca²⁺-triggered CV–CV fusion kinetics following 4 hour protease treatments. Papain, but not clostripain, inhibits response to 150 µM [Ca²⁺]_free. However, at 1 mM [Ca²⁺]_free, 100% fusion is observed in all cases. The decrease in the rate and extent of fusion at 150 µM [Ca²⁺]_free is consistent with a reduction in , as determined from a rightward shift in the Ca²⁺ activity curve. (B) Limiting papain treatment to 3 h alters the rate of fusion elicited with 150 µM [Ca²⁺]_free, with minimal change in the extent of fusion. Thus, the rate of fusion decreased without a substantial change in ; limited papain treatment decreased the fusion probability, p.