Previous studies have shown that protein disulfide isomerase (PDI), a member of the thioredoxin (TRX) superfamily, are broadly associated with immune responses in a variety of animals. However, it remains largely unknown about the direct roles of PDIs during a bacterial infection. In this study, we identified the presence of a single pdi gene in the amphioxus Branchiostoma japonicum, Bjpdi. The deduced protein BjPDI is structurally characterized by the presence of four Trx-like domains in the order of a, b, b’ and a’ and a short acidic C-terminal tail, that are characteristic of PDIs. We demonstrated that rBjPDI displayed both thiol reductase and disulfide bond isomerase activities, indicating comparability of BjPDI with PDIs in term of enzymatic activities. We also showed that rBjPDI induced bacterial agglutination and exhibited a lectin-like activity capable of binding both bacteria (E. coli and S. aureus) and their signature molecules LPS and LTA. Furthermore, BjPDI could kill S. aureus via inducing membrane depolarization and intracellular ROS production in vitro, and treatment of amphioxus with a blocking anti-PDI antibody in vivo markedly reduced the survival rate of amphioxus following attack by S. aureus. Collectively, our study demonstrates that amphioxus protein disulfide isomerase acts as both an enzyme and an immunocompotent factor, and reports the specific function and mode of action of PDIs in immune responses.

先前的研究表明，蛋白质二硫键异构酶 (PDI) 是硫氧还蛋白 (TRX) 超家族的成员，与多种动物的免疫反应广泛相关。然而，关于 PDI 在细菌感染期间的直接作用仍然很大程度上未知。在这项研究中，我们确定了文昌鱼Bjpdi中存在单个pdi基因。. 推导的蛋白质 BjPDI 的结构特征是存在四个 Trx 样结构域，顺序为 a、b、b' 和 a'，以及一个短的酸性 C 末端尾，这是 PDI 的特征。我们证明 rBjPDI 同时显示硫醇还原酶和二硫键异构酶活性，表明 BjPDI 与 PDI 在酶活性方面具有可比性。我们还表明，rBjPDI 诱导细菌凝集并表现出能够结合细菌（大肠杆菌和金黄色葡萄球菌）及其特征分子 LPS 和 LTA 的凝集素样活性。此外，BjPDI 可以通过在体外诱导膜去极化和细胞内 ROS 产生来杀死金黄色葡萄球菌，并且在体内用阻断性抗 PDI 抗体治疗文昌鱼显着降低了金黄色葡萄球菌攻击后文昌鱼的存活率。总的来说，我们的研究表明文昌鱼蛋白二硫键异构酶既是一种酶又是一种免疫活性因子，并报告了 PDI 在免疫反应中的特定功能和作用方式。