Deng Lunbin, Kaeser Pascal S, Xu Wei, Südhof Thomas C
Department of Molecular and Cellular Physiology, Stanford University, Lorry Lokey Building, 265 Campus Drive, Stanford, CA 94305-5453, USA.
Neuron. 2011 Jan 27;69(2):317-31. doi: 10.1016/j.neuron.2011.01.005.
At a synapse, the presynaptic active zone mediates synaptic vesicle exocytosis. RIM proteins are active zone scaffolding molecules that--among others--mediate vesicle priming and directly or indirectly interact with most other essential presynaptic proteins. In particular, the Zn²+ finger domain of RIMs binds to the C₂A domain of the priming factor Munc13, which forms a homodimer in the absence of RIM but a heterodimer with it. Here, we show that RIMs mediate vesicle priming not by coupling Munc13 to other active zone proteins as thought but by directly activating Munc13. Specifically, we found that the isolated Zn²+ finger domain of RIMs autonomously promoted vesicle priming by binding to Munc13, thereby relieving Munc13 homodimerization. Strikingly, constitutively monomeric mutants of Munc13 rescued priming in RIM-deficient synapses, whereas wild-type Munc13 did not. Both mutant and wild-type Munc13, however, rescued priming in Munc13-deficient synapses. Thus, homodimerization of Munc13 inhibits its priming function, and RIMs activate priming by disrupting Munc13 homodimerization.
在突触处，突触前活性区介导突触小泡胞吐作用。RIM蛋白是活性区支架分子，除其他作用外，它介导小泡引发，并直接或间接与大多数其他重要的突触前蛋白相互作用。特别地，RIMs的锌指结构域与引发因子Munc13的C₂A结构域结合，Munc13在没有RIM时形成同二聚体，但与RIM形成异二聚体。在此，我们表明RIMs介导小泡引发并非如之前所认为的那样通过将Munc13与其他活性区蛋白偶联，而是直接激活Munc13。具体而言，我们发现RIMs分离出的锌指结构域通过与Munc13结合自主促进小泡引发，从而解除Munc13的同二聚化。引人注目的是，Munc13的组成型单体突变体挽救了RIM缺陷型突触中的引发作用，而野生型Munc13则不能。然而，突变型和野生型Munc13都挽救了Munc13缺陷型突触中的引发作用。因此，Munc13的同二聚化抑制其引发功能，而RIMs通过破坏Munc13的同二聚化来激活引发作用。
