Ionic dependence of Ca²⁺ channel modulation by syntaxin 1A

Alteration of the kinetic properties of voltage-gated Ca²⁺ channels, Ca_v1.2 (Lc-type), Ca_v2.2 (N type), and Ca_v2.3 (R type), by syntaxin 1A (Syn1A) and synaptotagmin could modulate exocytosis. We tested how switching divalent charge carriers from Ca²⁺ to Sr²⁺ and Ba²⁺ affected Syn1A and synaptotagmin modulation of Ca²⁺-channel activation. Syn1A accelerated Ca_v1.2 activation if Ca²⁺ was the charge carrier; and by substituting for Ba²⁺, Syn1A slowed Ca_v1.2 activation. Syn1A also significantly accelerated Ca_v2.3 activation in Ca²⁺ and marginally in Ba²⁺. Synaptotagmin, on the other hand, increased the rate of activation of Ca_v2.3 and Ca_v2.2 in all permeating ions tested. The Syn1A-channel interaction, unlike the synaptotagmin-channel interaction, proved significantly more sensitive to the type of permeating ion. It is well established that exocytosis is affected by switching the charge carriers. Based on the present results, we suggest that the channel-Syn1A interaction could respond to the conformational changes induced within the channel during membrane depolarization and divalent ion binding. These changes could partially account for the charge specificity of synaptic transmission as well as for the fast signaling between the Ca²⁺ source and the fusion apparatus of channel-associated-vesicles (CAV). Furthermore, propagation of conformational changes induced by the divalent ions appear to affect the concerted interaction of the channel with the fusion/docking machinery upstream to free Ca²⁺ buildup and/or binding to a cytosolic Ca²⁺ sensor. These results raise the intriguing possibility that the channel is the Ca²⁺ sensor in the process of fast neurotransmitter release.