Ricin inhibits activation of the unfolded protein response by preventing splicing of the HAC1 mRNA
Ricin inhibits activation of the unfolded protein response by preventing splicing of the HAC1 mRNA
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DOI：
10.1074/jbc.m707981200
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发表时间：
2008-03-07
影响因子：
4.8
通讯作者：
Tumer, Nilgun E.
中科院分区：
文献类型：
--
作者：
Parikh, Bijal A.;Tortora, Andrew;Tumer, Nilgun E.
蓖麻毒素A链(RTA)通过从大rRNA中高度保守的α-sarcin/ricin环中移除特定的腺嘌呤来抑制蛋白质的合成。RTA在酵母中的表达具有其自身的信号序列，导致其转位到内质网(ER)并随后发生糖基化。由于RTA必须在内质网内展开，它可能容易受到宿主防御，如未折叠蛋白反应(UPR)。在表达活性位点突变体而不是野生型RTA的细胞中诱导了UPR，这表明RTA的活性位点在扰乱内质网应激反应中发挥了作用。没有信号序列的非活性RTA不能诱导UPR，这表明易位到内质网是诱导UPR的关键。野生型RTA抑制UPR的激活，不仅是由于蛋白质本身诱导的内质网应激，也是由于衣霉素和二硫苏糖醇等全局效应剂的作用。没有信号序列的成熟RTA也抑制了UPR，这为UPR的抑制发生在内质网的胞浆表面提供了证据。当以反式形式提供剪接形式的HAC1mRNA时，RTA不能抑制UPR，这表明它直接影响HAC1依赖的转录激活上游的UPR。在ER应激后表达RTA的细胞中仅检测到HAC1mRNA的前体形式，表明蓖麻毒素通过阻止HAC1mRNA的剪接而抑制UPR的激活。去除核糖体但不杀死细胞的RTA突变体不能抑制衣霉素激活的UPR，这提供了不能激活UPR以响应ER胁迫的证据，这是导致蓖麻毒素细胞毒性的原因之一。
Ricin A chain (RTA) inhibits protein synthesis by removing a specific adenine from the highly conserved alpha-sarcin/ricin loop in the large rRNA. Expression of RTA with its own signal sequence in yeast resulted in its translocation into the endoplasmic reticulum (ER) and subsequent glycosylation. Because RTA must unfold within the ER, it may be vulnerable to host defenses, such as the unfolded protein response (UPR). UPR was induced in cells expressing an active site mutant but not the wild type RTA, indicating that the active site of RTA played a role in perturbing the ER stress response. The inactive RTA without the signal sequence did not induce UPR, indicating that translocation into the ER was critical for induction of UPR. The wild type RTA inhibited activation of UPR not only due to ER stress induced by the protein itself but also by global effectors such as tunicamycin and dithiothreitol. Mature RTA without the signal sequence also inhibited UPR, providing evidence that inhibition of UPR occurred on the cytosolic face of the ER. RTA could not inhibit UPR when the spliced form of HAC1 mRNA was provided in trans, indicating that it had a direct effect on UPR upstream of HAC1-dependent transcriptional activation. Only the precursor form of HAC1 mRNA was detected in cells expressing RTA after exposure to ER stress, demonstrating that ricin inhibits activation of UPR by preventing HAC1 mRNA splicing. The RTA mutants that depurinated ribosomes but did not kill cells were not able to inhibit activation of UPR by tunicamycin, providing evidence that the inability to activate UPR in response to ER stress contributes to the cytotoxicity of ricin.