Richmond J E, Weimer R M, Jorgensen E M
Department of Biology, University of Utah, Salt Lake City 84112-0840, USA.
Nature. 2001 Jul 19;412(6844):338-41. doi: 10.1038/35085583.
The priming step of synaptic vesicle exocytosis is thought to require the formation of the SNARE complex, which comprises the proteins synaptobrevin, SNAP-25 and syntaxin. In solution syntaxin adopts a default, closed configuration that is incompatible with formation of the SNARE complex. Specifically, the amino terminus of syntaxin binds the SNARE motif and occludes interactions with the other SNARE proteins. The N terminus of syntaxin also binds the presynaptic protein UNC-13 (ref. 5). Studies in mouse, Drosophila and Caenorhabditis elegans suggest that UNC-13 functions at a post-docking step of exocytosis, most likely during synaptic vesicle priming. Therefore, UNC-13 binding to the N terminus of syntaxin may promote the open configuration of syntaxin. To test this model, we engineered mutations into C. elegans syntaxin that cause the protein to adopt the open configuration constitutively. Here we demonstrate that the open form of syntaxin can bypass the requirement for UNC-13 in synaptic vesicle priming. Thus, it is likely that UNC-13 primes synaptic vesicles for fusion by promoting the open configuration of syntaxin.
突触小泡胞吐作用的引发步骤被认为需要形成SNARE复合体，该复合体由突触小泡蛋白、SNAP-25和 syntaxin蛋白组成。在溶液中，syntaxin采取一种默认的封闭构象，这种构象与SNARE复合体的形成不兼容。具体而言，syntaxin的氨基末端结合SNARE模体并阻碍与其他SNARE蛋白的相互作用。syntaxin的N末端还结合突触前蛋白UNC-13（参考文献5）。在小鼠、果蝇和秀丽隐杆线虫中的研究表明，UNC-13在胞吐作用的对接后步骤发挥作用，最有可能是在突触小泡引发过程中。因此，UNC-13与syntaxin的N末端结合可能会促进syntaxin的开放构象。为了验证这一模型，我们对秀丽隐杆线虫的syntaxin进行了工程突变，使该蛋白组成性地采取开放构象。在此我们证明，syntaxin的开放形式可以绕过突触小泡引发过程中对UNC-13的需求。因此，UNC-13可能通过促进syntaxin的开放构象来使突触小泡做好融合的准备。
