Plant cysteine protease: multiple posttranslational processing and intracellular transport

在萌发的豌豆种子的子叶中表达一种液泡型半胱氨酸蛋白酶，命名为SH-EP，负责降解贮藏蛋白质。SH-EP是一种具有羧基末端内质网(ER)保留安全性KDEL的空泡蛋白水解酶。本文用7种抗体对芒果种子的子叶细胞进行了免疫细胞化学分析，以确定SH-EP从内质网到蛋白质储存液泡的胞内转运。内质网合成的SH-EP产物聚集在内质网中间区域的边缘，在那里形成运输小泡。含有大量Prosh-EP的囊泡称为KV，从内质网萌发出来，绕过高尔基复合体，被分类为蛋白质储存液泡。这种SH-EP通过KV的大量运输被认为是在萌发的种子的子叶细胞中介导了动态的蛋白质动员。我们讨论了KDEL末端的液泡半胱氨酸蛋白酶的KDEL序列作为内质网积累信号的可能性，以及由内质网衍生的KV样囊泡对这些蛋白酶的质量运输参与了植物的蛋白质动员。

A vacuolar cysteine proteinase, designated SH-EP, is expressed in the cotyledon of germinated Vigna mungo seeds, and is responsible for the degradation of storage proteins. SH-EP is a characteristic vacuolar proteinase possessing a carboxy-terminal endoplasmic reticulum (ER) retention secuence, KDEL. In the present work, immunocytochemical analysis of the cotyledon cells of V. mungo seeds was performed using seven kinds of antibodies to identify the intracellular transport of SH-EP from ER to protein storage vacuoles. A proform of SH-EP synthesized in ER accumulated at the edge of middle regions of ER where the transport vesicle was formed. The vesicle containing a large amount of proSH-EP, termed KV, budded off from ER, bypassed the Golgi complex and was sorted to protein storage vacuoles. This massive transport of SH-EP via KV was thought to mediate dynamic protein mobilization in the cotyledon cells of germinated seeds. We discuss the possibilities that the KDEL sequence of KDEL-tailed vacuolar cysteine proteinases functions as an accumulation signal at ER and that the mass transport of the proteinases by ER-derived KV-like vesicle is involved in the protein mobilization of plants.