Nucleus-encoded chloroplast proteins can be transported via the secretory pathway. The molecular mechanisms underlying the trafficking of chloroplast proteins between the intracellular compartments are largely unclear, and a cargo sorting receptor has not previously been identified in the secretory pathway. Here we report a cargo sorting receptor that is specifically present in Viridiplantae and mediates the transport of cargo proteins to the chloroplast. Using a forward genetic analysis, we identified a gene encoding a transmembrane protein (MtTP930) in barrel medic (Medicago truncatula). Mutation of MtTP930 resulted in impaired chloroplast function and a dwarf phenotype. MtTP930 is highly expressed in the aerial parts of the plant and is localized to the ER exit sites (ERESs) and Golgi. MtTP930 contains typical cargo sorting receptor motifs, interacts with Sar1, Sec12 and Sec24, and participates in coat protein II (COPII) vesicular transport. Importantly, MtTP930 can recognize the cargo proteins plastidial N-glycosylated nucleotide pyrophosphatase/ phosphodiesterase (MtNPP) and α-carbonic anhydrase (MtCAH) in the ER, and then transport them to the chloroplast via the secretory pathway. Mutation of a homolog of MtTP930 in Arabidopsis (Arabidopsis thaliana) resulted in a similar dwarf phenotype. Furthermore, MtNPP-GFP failed to localize to chloroplasts when transgenically expressed in Attp930 protoplasts, implying that these cargo sorting receptors are conserved in plants. These findings fill a gap in our understanding of the mechanism by which chloroplast proteins are sorted and transported via the secretory pathway.

中文翻译：

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货物分选受体通过分泌途径介导叶绿体蛋白运输


细胞核编码的叶绿体蛋白可以通过分泌途径运输。叶绿体蛋白在细胞内区室之间运输的分子机制在很大程度上尚不清楚，并且之前尚未在分泌途径中鉴定出货物分选受体。在这里，我们报道了一种货物分选受体，它特异存在于绿色植物中，介导货物蛋白向叶绿体的运输。通过正向遗传分析，我们在桶状苜蓿（Medicago truncatula）中鉴定出了编码跨膜蛋白（MtTP930）的基因。 MtTP930 突变导致叶绿体功能受损和矮化表型。 MtTP930 在植物的地上部分高度表达，并且定位于 ER 出口位点 (ERES) 和高尔基体。 MtTP930 包含典型的货物分选受体基序，与 Sar1、Sec12 和 Sec24 相互作用，并参与外壳蛋白 II (COPII) 囊泡运输。重要的是，MtTP930可以识别内质网中的货物蛋白质体N-糖基化核苷酸焦磷酸酶/磷酸二酯酶（MtNPP）和α-碳酸酐酶（MtCAH），然后通过分泌途径将它们转运至叶绿体。拟南芥 (Arabidopsis thaliana) 中 MtTP930 同源物的突变导致了类似的矮化表型。此外，当在 Attp930 原生质体中转基因表达时，MtNPP-GFP 未能定位到叶绿体，这意味着这些货物分选受体在植物中是保守的。这些发现填补了我们对叶绿体蛋白通过分泌途径分选和运输的机制的理解空白。