Cholera toxin up-regulates endoplasmic reticulum proteins that correlate with sensitivity to the toxin
Cholera toxin up-regulates endoplasmic reticulum proteins that correlate with sensitivity to the toxin
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DOI：
10.3181/0705-rm-132
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发表时间：
2008-02-01
影响因子：
3.2
通讯作者：
Draper, Rockford K.
中科院分区：
文献类型：
--
作者：
Dixit, Garima;Mikoryak, Carole;Draper, Rockford K.
霍乱毒素（CT）含有一条A链和五条B链。A链是一种共价修饰细胞质中三聚体G蛋白的酶，导致cAMP的过量产生。B链与鞘糖脂G（M1）结合，鞘糖脂G（M1）是CT的细胞表面受体，可启动受体介导的毒素内吞作用。胞吞作用后，CT通过逆行囊泡运输进入内质网（ER），其中A链逆行转位通过ER膜到达细胞质。逆易位机制知之甚少，但可能涉及ER应激反应的蛋白质，包括ER相关降解（ERAD）途径。我们在这里报告说，用CT或CTB处理细胞可以快速上调BiP、Derlin-1和Derlin-2的水平，它们是ER应激反应和ERAD的已知参与者。CT没有诱导钙连接蛋白，另一种已知的ER应激反应，表明CT介导的ER蛋白诱导在这个时间范围内是选择性的。这些数据表明，CT可能通过快速上调一组参与逆向易位过程的ER蛋白来促进A链向细胞质的逆向易位。为了支持这一观点，诱导BiP、Derlin-1和Derlin-2的各种条件使细胞对CT敏感，而抑制其诱导的条件使细胞对CT不敏感。此外，用siRNA特异性抑制Derlin-1可保护细胞免受CT。此外，使用抗CTA或抗CTB抗体，Derlin-1与来自CT处理的细胞的CTA或CTB共免疫沉淀。总之，结果与CT的B链上调ER蛋白的假设一致，这可能有助于A链穿过ER膜的逆易位。
Cholera toxin (CT) contains one A chain and five B chains. The A chain is an enzyme that covalently modifies a trimeric G protein in the cytoplasm, resulting in the overproduction of cAMP. The B chain binds the glycosphingolipid G(M1), the cell surface receptor for CT, which initiates receptor-mediated endocytosis of the toxin. After endocytosis, CT enters the endoplasmic reticulum (ER) via retrograde vesicular traffic where the A chain retrotranslocates through the ER membrane to reach the cytoplasm. The retro-translocation mechanism is poorly understood, but may involve proteins of the ER stress response, including the ER associated degradation (ERAD) pathway. We report here that treating cells with CT or CTB quickly up-regulates the levels of BiP, Derlin-1, and Derlin-2, known participants in the ER stress response and ERAD. CT did not induce calnexin, another known responder to ER stress, indicating that the CT-mediated induction of ER proteins is selective in this time frame. These data suggest that CT may promote retro-translocation of the A chain to the cytoplasm by rapidly up-regulating a set of ER proteins involved in the retro-translocation process. In support of this idea, a variety of conditions that induced BiP, Derlin-1, and Derlin-2 sensitized cells to CT and conditions that inhibited their induction de-sensitized cells to CT. Moreover, specifically suppressing Derlin-1 with siRNA protected cells from CT. In addition, Derlin-1 co-immunoprecipitated with CTA or CTB from CT-treated cells using anti-CTA or anti-CTB antibodies. Altogether, the results are consistent with the hypothesis that the B chain of CT up-regulates ER proteins that may assist in the retro-translocation of the A chain across the ER membrane.