PKC-induced Sensitization of Ca²⁺-dependent Exocytosis Is Mediated by Reducing the Ca²⁺ Cooperativity in Pituitary Gonadotropes

The highly cooperative nature of Ca²⁺-dependent exocytosis is very important for the precise regulation of transmitter release. It is not known whether the number of binding sites on the Ca²⁺ sensor can be modulated or not. We have previously reported that protein kinase C (PKC) activation sensitizes the Ca²⁺ sensor for exocytosis in pituitary gonadotropes. To further unravel the underlying mechanism of how the Ca²⁺ sensor is modulated by protein phosphorylation, we have performed kinetic modeling of the exocytotic burst and investigated how the kinetic parameters of Ca²⁺-triggered fusion are affected by PKC activation. We propose that PKC sensitizes exocytosis by reducing the number of calcium binding sites on the Ca²⁺ sensor (from three to two) without significantly altering the Ca²⁺-binding kinetics. The reduction in the number of Ca²⁺-binding steps lowers the threshold for release and up-regulates release of fusion-competent vesicles distant from Ca²⁺ channels.