Molecular Mechanisms of Synaptic Vesicle Fusion

Synaptopagmin I(SYT I)计划；SYT I被广泛认为是一种主要的钙递质快速释放感受器，它有两个可能的钙结合结构域，C2a和C2b。为了阐明其在突触原位传递中的作用，我们研究了具有syt I基因突变的果蝇胚胎神经肌肉接头(NMJ)的突触电流。突触电流是由神经刺激在含有不同浓度的Ca~(2+)和Gt~(2+)的外部溶液或高K~(++)溶液中诱导的。在一个零等位基因syt1^&lt；ad4&gt；中，同步突触电流很少被诱发，但并未被消除。其数量含量随[Ca~(2+)]的增加而增加，在双对数曲线上斜率为0.7，而对照为2.7。缺少完整C2B的等位基因syt1；ad1&gt；的斜率为0.9，与syt1；lt；ad4&gt；的斜率相同，而C2B中只有一个氨基酸取代的等位基因syt1^&lt；ad3&gt；的斜率为1.6。在生理盐水中，SytI、Ad4和Gt；与其他sytI等位基因或对照组的微小突触电流频率无明显差异。考虑到Syt1；AD4&gt；的高渗性和钙离子载体的反应要小得多，这一发现表明，在没有SytI的情况下，停靠/预充的囊泡自发释放的几率更高。在高K^+盐度下，syt1；AD4&gt；的微小频率对[Ca^&lt；2+&gt；]的依赖程度比对照组小，而在sytI^&lt；AD3&gt；的小频率对[Ca^&lt；]的依赖程度甚至低于sytI^&lt；AD3&gt；。我们认为，果蝇SYT I是一种钙离子同步释放感受器，但抑制自发量子释放。

Synaptotagmin I (Syt I) project ; Syt I is widely considered to be a major Ca^<2+> sensor for fast transmitter release and has two putative Ca^<2+> binding domains, C2A and C2B. To elucidate its roles in synaptic transmission in situ, we studied synaptic currents at the neuromuscular junction (NMJ) of Drosophila embryos that have mutations in syt I. Synaptic currents were induced by nerve-stimulation in external solutions containing various concentrations of Ca^<2+> or by high K^+ solutions. In a null allele, syt I^<AD4>, synchronized synaptic currents were rarely evoked but not abolished. Its quantal content increased with [Ca^<2+>] with a slope of 0.7 in double logarithmic plot, in contrast to 2.7 in control. The slope of 0.9 in an allele that lacks entire C2B, syt I^<AD1>, was not different than in syt I^<AD4>, whereas in another allele, syt I^<AD3>, with one amino-acid substitution in C2B, it was 1.6. In normal saline, the miniature synaptic current (mini) frequency in syt I^<AD4> was not different than in other syt I alleles or in controls. Considering much smaller hypertonicity and Ca^<2+>-ionophore responses in syt I^<AD4>, this finding suggests that the release probability of docked/primed vesicles for spontaneous release is higher in the absence of Syt I. In high K^+ saline, the dependency of mini frequency on [Ca^<2+>] in syt I^<AD4> was less than that in controls, while that in syt I^<AD3> was even lower than in syt I^<AD4>. We conclude that Drosophila Syt I is a Ca^<2+> sensor for synchronized release but inhibits spontaneous quantal release.