Syntaxin Modulation of Slow Inactivation of N-Type Calcium Channels

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Abstract
Syntaxin, a membrane protein vital in triggering vesicle fusion, interacts with voltage-gated N- and P/Q-type Ca2+channels. This biochemical association is proposed to colocalize Ca2+ channels and presynaptic release sites, thus supporting rapid and efficient initiation of neurotransmitter release. The syntaxin channel interaction may also support a novel signaling function, to modulate Ca2+ channels according to the state of the associated release machinery (Bezprozvanny et al., 1995;Wiser et al., 1996; see also Mastrogiacomo et al., 1994). Here we report that syntaxin 1A (syn1A) coexpressed with N-type channels inXenopus oocytes greatly promoted slow inactivation gating, but had little or no effect on the onset of and recovery from fast inactivation. Accordingly, the effectiveness of syntaxin depended strongly on voltage protocol. Slow inactivation was found for N-type channels even in the absence of syntaxin and could be distinguished from fast inactivation on the basis of its slow kinetics, distinct voltage dependence (voltage-independent at potentials higher than the level of half-inactivation), and temperature independence (Q10, ∼0.8). Trains of action potential-like stimuli were more effective than steady depolarizations in stabilizing the slowly inactivated condition. Agents that stimulate protein kinase C decreased the inhibitory effect of syntaxin on N-type channels. Application of BoNtC1 to cleave syntaxin sharply attenuated the modulatory effects on Ca2+ channel gating, consistent with structural analysis of syntaxin modulation, supporting use of this toxin to test for the impact of syntaxin on Ca2+ influx in nerve terminals.