Plant cysteine protease: multiple posttranslational processing and intracellular transport

一种被称为SH-EP的空泡半胱氨酸蛋白酶在萌发的芒戈种子子叶中表达，并负责贮藏蛋白的降解。SH-EP是一种具有羧基末端内质网保留序列（KDEL）的特征性空泡蛋白酶。本研究利用7种抗体对芒戈种子子叶细胞进行免疫细胞化学分析，以鉴定SH-EP从内质网到蛋白储存液泡的细胞内转运。在内质网中合成的SH-EP形式积聚在内质网中间区域的边缘，在那里形成了运输囊泡。含有大量proSH-EP的囊泡，称为KV，从内质网脱离，绕过高尔基复合体，被分类为蛋白质储存液泡。SH-EP通过KV的大量转运被认为介导了萌发种子子叶细胞的动态蛋白质动员。我们讨论了KDEL尾空泡型半胱氨酸蛋白酶的KDEL序列作为内质网积累信号的可能性，以及内质网衍生的kv样囊泡对蛋白酶的质量运输参与植物蛋白动员的可能性。

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.