Department of Neuroscience, University of Texas Southwestern Medical Center, Dallas, Texas, USA.
Nat Struct Mol Biol. 2010 Mar;17(3):280-8. doi: 10.1038/nsmb.1758. Epub 2010 Feb 14.
Munc13 is a multidomain protein present in presynaptic active zones that mediates the priming and plasticity of synaptic vesicle exocytosis, but the mechanisms involved remain unclear. Here we use biophysical, biochemical and electrophysiological approaches to show that the central C(2)B domain of Munc13 functions as a Ca(2+) regulator of short-term synaptic plasticity. The crystal structure of the C(2)B domain revealed an unusual Ca(2+)-binding site with an amphipathic alpha-helix. This configuration confers onto the C(2)B domain unique Ca(2+)-dependent phospholipid-binding properties that favor phosphatidylinositolphosphates. A mutation that inactivated Ca(2+)-dependent phospholipid binding to the C(2)B domain did not alter neurotransmitter release evoked by isolated action potentials, but it did depress release evoked by action-potential trains. In contrast, a mutation that increased Ca(2+)-dependent phosphatidylinositolbisphosphate binding to the C(2)B domain enhanced release evoked by isolated action potentials and by action-potential trains. Our data suggest that, during repeated action potentials, Ca(2+) and phosphatidylinositolphosphate binding to the Munc13 C(2)B domain potentiate synaptic vesicle exocytosis, thereby offsetting synaptic depression induced by vesicle depletion.
Munc13 是一种存在于突触前活性区的多功能蛋白，它介导突触囊泡胞吐作用的引发和可塑性，但涉及的机制仍不清楚。在这里，我们使用生物物理、生化和电生理方法表明 Munc13 的中心 C(2)B 结构域作为 Ca(2+)调节短期突触可塑性。C(2)B 结构域的晶体结构揭示了一个具有两亲性α-螺旋的不寻常的 Ca(2+)结合位点。这种构象赋予了 C(2)B 结构域独特的 Ca(2+)依赖性磷脂结合特性，有利于磷脂酰肌醇磷酸。一种使 C(2)B 结构域的 Ca(2+)依赖性磷脂结合失活的突变不会改变由孤立动作电位引发的神经递质释放，但它确实会抑制由动作电位串引发的释放。相比之下，一种增加 C(2)B 结构域与 Ca(2+)依赖性磷脂酰肌醇双磷酸结合的突变增强了由孤立动作电位和动作电位串引发的释放。我们的数据表明，在重复的动作电位期间，Ca(2+)和磷脂酰肌醇磷酸与 Munc13 C(2)B 结构域结合增强了突触囊泡胞吐作用，从而抵消了由于囊泡耗竭引起的突触抑制。
