In rice (Oryza sativa) endosperm cells, mRNAs encoding glutelin and prolamine are translated on distinct cortical-endoplasmic reticulum (ER) subdomains (the cisternal-ER and protein body-ER), a process that facilitates targeting of their proteins to different endomembrane compartments. Although the cis- and trans-factors responsible for mRNA localization have been defined over the years, how these mRNAs are transported to the cortical ER has yet to be resolved. Here, we show that the two interacting glutelin zipcode RNA binding proteins (RBPs), RBP-P and RBP-L, form a quaternary complex with the membrane fusion factors n-ethylmaleimide-sensitive factor (NSF) and the small GTPase Rab5a, enabling mRNA transport on endosomes. Direct interaction of RBP-L with Rab5a, between NSF and RBP-P, and between NSF and Rab5a, were established. Biochemical and microscopic analyses confirmed the co-localization of these RBPs with NSF on Rab5a-positive endosomes that carry glutelin mRNAs. Analysis of a loss-of-function rab5a mutant showed that glutelin mRNA and the quaternary complex were mis-targeted to the extracellular paramural body structure formed by aborted endosomal trafficking, further confirming the involvement of endosomal trafficking in glutelin mRNA transport. Overall, these findings demonstrate that mRNA localization in plants co-opts membrane trafficking via the acquisition of new functional binding properties between RBPs and two essential membrane trafficking factors, thus defining an endosomal anchoring mechanism in mRNA localization.

在水稻 ( Oryza sativa ) 胚乳细胞中，编码谷蛋白和谷醇溶蛋白的 mRNA 在不同的皮质-内质网 (ER) 子域（池-ER 和蛋白体-ER）上翻译，这一过程有助于将其蛋白质靶向不同的内膜区室。尽管负责 mRNA 定位的顺式和反式因子多年来已被确定，但这些 mRNA 如何转运至皮质内质网仍有待解决。在这里，我们发现两种相互作用的谷蛋白邮政编码 RNA 结合蛋白 (RBP) RBP-P 和 RBP-L 与膜融合因子 n-乙基马来酰亚胺敏感因子 (NSF) 和小 GTPase Rab5a 形成四元复合物，从而使mRNA在内体上的运输。建立了 RBP-L 与 Rab5a、NSF 与 RBP-P 之间以及 NSF 与 Rab5a 之间的直接相互作用。生化和显微镜分析证实这些 RBP 与 NSF 共定位于携带谷蛋白 mRNA 的 Rab5a 阳性内体上。对功能缺失的rab5a突变体的分析表明，谷蛋白 mRNA 和四元复合物被错误定位到由内体运输中止形成的细胞外旁体结构，进一步证实了内体运输参与了谷蛋白 mRNA 运输。总的来说，这些发现表明，植物中的 mRNA 定位通过获得 RBP 和两个重要膜运输因子之间的新功能结合特性来选择膜运输，从而定义了 mRNA 定位中的内体锚定机制。