Department of Chemistry, University of Utah, Salt Lake City, UT 84112, USA.
Toxins (Basel). 2023 Aug 2;15(8):486. doi: 10.3390/toxins15080486.
Mycolactone is an exotoxin produced by that causes the neglected tropical skin disease Buruli ulcer. This toxin inhibits the Sec61 translocon in the endoplasmic reticulum (ER), preventing the host cell from producing several secretory and transmembrane proteins, resulting in cytotoxic and immunomodulatory effects. Interestingly, only one of the two dominant isoforms of mycolactone is cytotoxic. Here, we investigate the origin of this specificity by performing extensive molecular dynamics (MD) simulations with enhanced free energy sampling to query the association trends of the two isoforms with both the Sec61 translocon, using two distinct cryo-electron microscopy (cryo-EM) models as references, and the ER membrane, which serves as a toxin reservoir prior to association. Our results suggest that mycolactone B (the cytotoxic isoform) has a stronger association with the ER membrane than mycolactone A due to more favorable interactions with membrane lipids and water molecules. This could increase the reservoir of toxin proximal to the Sec61 translocon. In one model of Sec61 inhibited by mycolactone, we find that isomer B interacts more closely with residues thought to play a key role in signal peptide recognition and, thus, are essential for subsequent protein translocation. In the other model, we find that isomer B interacts more closely with the lumenal and lateral gates of the translocon, the dynamics of which are essential for protein translocation. These interactions induce a more closed conformation, which has been suggested to block signal peptide insertion and subsequent protein translocation. Collectively, these findings suggest that isomer B's unique cytotoxicity is a consequence of both increased localization to the ER membrane and channel-locking association with the Sec61 translocon, facets that could be targeted in the development of Buruli Ulcer diagnostics and Sec61-targeted therapeutics.
Mycolactone 是一种由 产生的外毒素，可导致被忽视的热带皮肤疾病——布鲁里溃疡。这种毒素抑制内质网 (ER) 中的 Sec61 易位体，阻止宿主细胞产生几种分泌和跨膜蛋白，从而产生细胞毒性和免疫调节作用。有趣的是，mycolactone 的两种主要同工型中只有一种具有细胞毒性。在这里，我们通过进行广泛的分子动力学 (MD) 模拟和增强自由能采样来研究这种特异性的起源，以查询两种同工型与 Sec61 易位体的结合趋势，使用两个不同的低温电子显微镜 (cryo-EM) 模型作为参考，以及 ER 膜，它在与 Sec61 易位体结合之前充当毒素储库。我们的结果表明，mycolactone B（细胞毒性同工型）与 ER 膜的结合比 mycolactone A 更强，因为与膜脂质和水分子的相互作用更有利。这可能会增加靠近 Sec61 易位体的毒素储库。在一个被 mycolactone 抑制的 Sec61 模型中，我们发现异构体 B 与被认为在信号肽识别中起关键作用的残基相互作用更紧密，因此对于随后的蛋白易位至关重要。在另一个模型中，我们发现异构体 B 与易位体的腔和侧门更紧密地相互作用，这些门的动力学对于蛋白易位至关重要。这些相互作用诱导更封闭的构象，据推测，这种构象会阻止信号肽插入和随后的蛋白易位。总的来说，这些发现表明异构体 B 的独特细胞毒性是其在 ER 膜上的定位增加和与 Sec61 易位体的通道锁定结合的结果，这些方面可能成为开发布鲁里溃疡诊断和 Sec61 靶向治疗的目标。
