Ionic dependence of Ca2+ channel modulation by syntaxin 1A

Alteration of the kinetic properties of voltage-gated Ca2+ channels, Ca(v)1.2 (Lc-type), Ca(v)2.2 (N type), and Ca(v)2.3 (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 Ca(2+)channel activation. Syn1A accelerated Ca(v)1.2 activation if Ca2+ was the charge carrier; and by substituting for Ba2+, Syn1A slowed Ca(v)1.2 activation. Syn1A also significantly accelerated Ca(v)2.34 activation in Ca2+ and marginally in Ba2+. Synaptotagmin, on the other hand, increased the rate of activation of Ca(v)2.3 and Ca(v)2.2 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.