Lu Pin, Li Huanhuan, Li Ning, Singh Ravi N, Bishop Colin E, Chen Xiangxian, Lu Baisong
Anhui Normal University, Wuhu, China.
Institute for Regenerative Medicine, Wake Forest University Health Sciences, Winston Salem, North Carolina, United States of America.
PLoS One. 2017 Oct 5;12(10):e0185992. doi: 10.1371/journal.pone.0185992. eCollection 2017.
Some RNA species, especially microRNAs, are non-randomly sorted into exosomes, but how selectivity of RNA exosomal sorting is achieved is unknown. We found that all three variants of RNA-binding ubiquitin E3 ligase (MEX3C)-MEX3C-1, MEX3C-2, and MEX3C-3 -interact with adaptor-related protein complex 2 (AP-2), a cargo adaptor in clathrin-mediated endocytosis. MEX3C's C-terminal RING finger domain and the hnRNP K homology (KH) domain shared by the three MEX3C variants are both necessary for MEX3C/AP-2 interaction. MEX3C associates with the endolysosomal compartment through an endocytosis-like process. siRNA-mediated inhibition of the MEX3C or AP-2 complex substantially decreased exosomal but not cellular microRNA miR-451a expression. Exosomal sorting is ceramide-dependent but not ESCRT-dependent in microRNA miR-451a. That RNA-binding protein associates with membrane trafficking machinery, and that its involvement in exosomal microRNA expression, suggest the existence of a mechanism for specific recruiting of RNA molecules to endosomes for subsequent exosomal sorting.
一些RNA种类，尤其是微小RNA，会非随机地分选到外泌体中，但RNA外泌体分选的选择性是如何实现的尚不清楚。我们发现RNA结合泛素E3连接酶（MEX3C）的所有三种变体——MEX3C-1、MEX3C-2和MEX3C-3——都与衔接蛋白相关蛋白复合物2（AP-2）相互作用，AP-2是网格蛋白介导的内吞作用中的一种货物衔接蛋白。MEX3C的C末端环指结构域以及三种MEX3C变体共有的异质性核糖核蛋白K同源性（KH）结构域对于MEX3C/AP-2相互作用都是必需的。MEX3C通过类似内吞作用的过程与内溶酶体区室结合。小干扰RNA介导的对MEX3C或AP-2复合物的抑制显著降低了外泌体中而非细胞中的微小RNA miR-451a的表达。在微小RNA miR-451a中，外泌体分选是神经酰胺依赖性的而非内体分选转运复合物（ESCRT）依赖性的。RNA结合蛋白与膜运输机制相关联，并且其参与外泌体微小RNA表达，这表明存在一种将RNA分子特异性招募到内体以便随后进行外泌体分选的机制。
