State Key Laboratory of Plant Physiology and Biochemistry and National Maize Improvement Center, Department of Plant Genetics and Breeding, China Agricultural University, Beijing, 100193, P. R. China.
Engineering Research Center of Plant Growth Regulator, Ministry of Education & College of Agronomy and Biotechnology, China Agricultural University, Beijing 100193, P. R. China.
Plant Physiol. 2021 Apr 2;185(3):985-1001. doi: 10.1093/plphys/kiaa060.
Endoplasmic reticulum (ER) type I signal peptidases (ER SPases I) are vital proteases that cleave signal peptides from secreted proteins. However, the specific function of ER SPase I in plants has not been genetically characterized, and the substrate is largely unknown. Here, we report the identification of a maize (Zea mays) miniature seed6 (mn6) mutant. The loss-of-function mn6 mutant exhibited severely reduced endosperm size. Map-based cloning and molecular characterization indicated that Mn6 is an S26-family ER SPase I, with Gly102 (box E) in Mn6 critical for protein function during processing. Mass spectrometric and immunoprecipitation analyses revealed that Mn6 is predominantly involved in processing carbohydrate synthesis-related proteins, including the cell wall invertase miniature seed1 (Mn1), which is specifically expressed in the basal endosperm transfer layer. RNA and protein expression levels of Mn1 were both significantly downregulated in the mn6 mutant. Due to the significant reduction in cell wall invertase activity in the transfer cell layer, mutation of Mn6 caused dramatic defects in endosperm development. These results suggest that proper maturation of Mn1 by Mn6 may be a crucial step for proper seed filling and maize development.
内质网（ER）I 型信号肽酶（ER SPase I）是一种重要的蛋白酶，能够从分泌蛋白中切割信号肽。然而，ER SPase I 在植物中的具体功能尚未通过遗传方法进行表征，其底物也很大程度上未知。在这里，我们报道了一个玉米（Zea mays）小粒 6 号（mn6）突变体的鉴定。mn6 功能丧失型突变体表现出严重的胚乳大小减小。基于图谱的克隆和分子特征表明，Mn6 是 S26 家族 ER SPase I，其 Gly102（框 E）在 Mn6 的加工过程中对蛋白功能至关重要。质谱和免疫沉淀分析表明，Mn6 主要参与碳水化合物合成相关蛋白的加工，包括特异性表达在基础胚乳转移层的细胞壁转化酶小粒 1（Mn1）。mn6 突变体中 Mn1 的 RNA 和蛋白表达水平均显著下调。由于细胞壁转化酶活性在转移细胞层中显著降低，Mn6 的突变导致胚乳发育的严重缺陷。这些结果表明，Mn6 对 Mn1 的适当成熟可能是适当种子填充和玉米发育的关键步骤。
