Syntaxin modulation of slow inactivation of N-type calcium channels

Syntaxin, a membrane protein vital in triggering vesicle fusion, interacts with voltage-gated N- and P/Q-type Ca2+ channels. This biochemical associat ion 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 functi on, to modulate Ca2+ channels according to the state of the associated rele ase machinery (Bezprozvanny et al., 1995; Wiser et al., 1996; see also Mast rogiacomo et al., 1994). Here we report that syntaxin 1A (syn1A) coexpresse d with N- type channels in Xenopus oocytes greatly promoted slow inactivati on gating, but had little or no effect on the onset of and recovery from fa st inactivation. Accordingly, the effectiveness of syntaxin depended strong ly on voltage protocol. Slow inactivation was found for N- type channels ev en in the absence of syntaxin and could be distinguished from fast inactiva tion on the basis of its slow kinetics, distinct voltage dependence (voltag e-independent at potentials higher than the level of half-inactivation), an d temperature independence (Q(10), similar to 0.8). Trains of action potent ial-like stimuli were more effective than steady depolarizations in stabili zing the slowly inactivated condition. Agents that stimulate protein kinase C decreased the inhibitory effect of syntaxin on N- type channels. Applica tion of BoNtCl to cleave syntaxin sharply attenuated the modulatory effects on Ca2+ channel gating, consistent with structural analysis of syntaxin mo dulation, supporting use of this toxin to test for the impact of syntaxin o n Ca2+ influx in nerve terminals.