Frontier Research Institute for Interdisciplinary Sciences and Graduate School of Life Sciences, Tohoku University, Aramaki Aza Aoba 6-3, Aoba-ku, Sendai, Miyagi 980-8578, Japan.
Howard Hughes Medical Institute, Department of Biology, Stanford University, 385 Serra Mall, Stanford, CA 94305, USA.
Curr Biol. 2017 Sep 11;27(17):2569-2578.e4. doi: 10.1016/j.cub.2017.07.013. Epub 2017 Aug 17.
Axonal transport of synaptic vesicle precursors (SVPs) is essential for synapse development and function. The conserved ARF-like small GTPase ARL-8 is localized to SVPs and directly activates UNC-104/KIF1A, the axonal-transport kinesin for SVPs in C. elegans. It is not clear how ARL-8 is activated in this process. Here we show that part of the BLOC-1-related complex (BORC), previously shown to regulate lysosomal transport, is required to recruit and activate ARL-8 on SVPs. We found mutations in six BORC subunits-blos-1/BLOS1, blos-2/BLOS2, snpn-1/Snapin, sam-4/Myrlysin, blos-7/Lyspersin, and blos-9/MEF2BNB-cause defects in axonal transport of SVPs, leading to ectopic accumulation of synaptic vesicles in the proximal axon. This phenotype is suppressed by constitutively active arl-8 or unc-104 mutants. Furthermore, SAM-4/Myrlysin, a subunit of BORC, promotes the GDP-to-GTP exchange of ARL-8 in vitro and recruits ARL-8 onto SVPs in vivo. Thus, BORC regulates the axonal transport of synaptic materials and synapse formation by controlling the nucleotide state of ARL-8. Interestingly, the other two subunits of BORC essential for lysosomal transport, kxd-1/KXD1 and blos-8/Diaskedin, are not required for the SVP transport, suggesting distinct subunit requirements for lysosomal and SVP trafficking.
突触小泡前体（SVPs）的轴突运输对于突触的发育和功能至关重要。保守的 ARF 样小分子 GTP 酶 ARL-8 定位于 SVPs 上，并直接激活 UNC-104/KIF1A，这是秀丽隐杆线虫中 SVPs 的轴突运输驱动蛋白。目前尚不清楚该过程中 ARL-8 是如何被激活的。在这里，我们发现先前被证明调节溶酶体运输的 BLOC-1 相关复合物（BORC）的一部分，需要招募和激活 SVPs 上的 ARL-8。我们发现 BORC 的六个亚基（blos-1/BLOS1、blos-2/BLOS2、snpn-1/Snapin、sam-4/Myrlysin、blos-7/Lyspersin 和 blos-9/MEF2BNB）的突变导致 SVPs 的轴突运输缺陷，导致突触囊泡在近端轴突中异常积累。这种表型被组成性激活的 arl-8 或 unc-104 突变体所抑制。此外，BORC 的一个亚基 SAM-4/Myrlysin 在体外促进 ARL-8 的 GDP 到 GTP 交换，并在体内将 ARL-8 招募到 SVPs 上。因此，BORC 通过控制 ARL-8 的核苷酸状态来调节突触物质的轴突运输和突触形成。有趣的是，BORC 中对溶酶体运输至关重要的另外两个亚基 kxd-1/KXD1 和 blos-8/Diaskedin 对于 SVP 运输不是必需的，这表明溶酶体和 SVP 运输对亚基有不同的要求。
