Study on the proprotein-converting enzyme in the Golgi Apparatus

1. Furin被发现定位于高尔基体，当在COS细胞中与原白蛋白或与原C3共表达时，Furin可促进原白蛋白和补体第三组分（C3）的形式裂解为成熟的白蛋白和C3。在这项研究中，我们进一步证明了纯化的可溶性furin，其中假定的跨膜和细胞质尾部被删除，能够在体外将前体转化为成熟的蛋白质，这表明furin是在高尔基体中寻找已久的加工蛋白酶的良好候选。当肝细胞或HepG2细胞在细胞内钙离子耗尽的条件下培养时，发现分泌原白蛋白和原c3而不是成熟蛋白。补体第四组分的前体也从缺钙的HepG2细胞分泌到培养基中。这些结果提示钙离子依赖性蛋白酶参与血浆蛋白前体的加工。

1. Furin is found to be localized at the Golgi apparatus and enhance the cleavage of both pro-albumin and the proform of the third component of complement (C3) into mature albumin and C3, when coexpressed in the COS cells either with proalbumin or with pro-C3. In this study we further showed that the purified soluble furin from which the putative trans-membrane and cytoplasmic tail were deleted, is able to convert the precursors into mature proteins in vitro, suggesing that furin is a good candidate a long-sought processing protease at the Golgi apparatus.2. When hepatocytes or HepG2 cells were cultured under the conditions where intracellular calcium ion was depleted, proalbumin and pro-C3 were found to be secreted instead of mature proteins. The precursor of the fourth component of complement was also secreted into the medium from the calcium-depleted HepG2 cells. These results suggest that calcium ion-dependent protease is involved in the processing of plasma protein precursors.