Intracellular transport and processing of plant proteases

SH-EP是一种来自萌发的绿豆(Vigna Mungo)子叶的半胱氨酸蛋白酶，其功能是降解液泡储存蛋白，为新植物生长的早期阶段提供氮和碳。虽然SH-EP似乎是木瓜蛋白酶超家族半胱氨酸蛋白酶的典型成员，但Prosh-EP的cDNA表明它也具有一个在ER腔驻留蛋白上发现的羧基末端ER保留序列KDEL。为了研究ER保留序列的功能，我们利用杆状病毒系统在SF-9细胞中表达了SH-EP的全长cDNA和负的KDEL对照。我们对SH-EP在SF-9细胞中的合成、加工和运输的观察支持一个模型，即KDEL ER-保留序列在内质网内或退出内质网后立即被翻译后移除，导致Prosh-EP积累减去其KDEL信号。正是这种中间形式似乎通过内膜系统进行，并随后成为…KDEL ER-保留序列的作用可能是延缓Prosh-EP从内质网退出，导致其在ER内暂时积累，使Prosh-EP及其KDEL保留序列成为瞬时酶原。SF-9细胞中的这种翻译后处理模式解释了先前从绿豆种子中纯化的成熟SH-EP的羧基末端测序结果，该结果被证明缺乏KDEL。在种子叶细胞中，SH-EP及其KDEL可能通过允许合成和储存足够的Prosh-EP来提供对液泡蛋白降解的翻译后控制，从而允许细胞调节储存的非活性形式的Prosh-EP向其激活部位和底物的输送，从而快速降解储存的蛋白质。ER保留序列的去除具有调节蛋白质转运到功能部位的潜力，除了在维持ER管腔的蛋白质组成方面具有公认的作用外，它还提供了ER保留序列的另一种作用。较少

SH-EP is a cysteine protease from germinating mung bean (Vigna mungo) cotyledons that functions to degrade vacuolar storage proteins to supply nitrogen and carbon for the early stages of the new plant's growth. Although SH-EP appears to be a typical member of the papain superfamily cysteine proteases the cDNA of proSH-EP shows it also possesses a carboxyterminal ER retention sequence KDEL found on ER-lumen resident proteins. In order to examine the function of the ER retention sequence we expressed a full-length cDNA of SH-EP and a minus-KDEL control in Sf-9 cells using the baculovirus sytem. Our observations on the synthesis, processing and trafficking of SH-EP in Sf-9 cells support a model where the KDEL ER-retention sequence is posttranslationally removed either within the ER or immediately after its exit from the ER resulting in the accumulation of proSH-EP minus its KDEL signal. It is this intermediate form that appears to progress through the endomembrane system and is subsequent … More ly processed to form mature active SH-EP.The function of the KDEL ER-rentention sequence may be to retard the exit of proSH-EP from the ER leading to its temporary accumulation within the ER making proSH-EP with its KDEL retention sequence a transient zymogen. This pattern of posttranslational processing in Sf-9 cells provides an explaination for previous results on the carboxyterminal sequenching of mature SH-EP purified from mung bean seeds which was shown to lack the KDEL.SH-EP with its KDEL in the seed cotyledon cells may serve to provide a posttranslational control of vacuole protein degradation by allowing sufficient proSH-EP to be synthesized and stored to rapidly degrade storage proteins by allowing the cell to regulate the delivery of the stored inactive form of proSH-EP to its activation site and substrate. The removal of an ER retention sequence with its potential to regulate protein delivery to a functional site presents an alternative role for ER retention sequences in addition to its well established role in maintaining the protein composition of the ER lumen. Less