Department of Microbial Pathogenesis, School of Dentistry, University of Maryland, Baltimore, 650 W. Baltimore Street, Baltimore, MD 21201, USA.
J Mol Biol. 2024 Apr 15;436(8):168521. doi: 10.1016/j.jmb.2024.168521. Epub 2024 Mar 7.
Dedicated translocase channels are nanomachines that often, but not always, unfold and translocate proteins through narrow pores across the membrane. Generally, these molecular machines utilize external sources of free energy to drive these reactions, since folded proteins are thermodynamically stable, and once unfolded they contain immense diffusive configurational entropy. To catalyze unfolding and translocate the unfolded state at appreciable timescales, translocase channels often utilize analogous peptide-clamp active sites. Here we describe how anthrax toxin has been used as a biophysical model system to study protein translocation. The tripartite bacterial toxin is composed of an oligomeric translocase channel, protective antigen (PA), and two enzymes, edema factor (EF) and lethal factor (LF), which are translocated by PA into mammalian host cells. Unfolding and translocation are powered by the endosomal proton gradient and are catalyzed by three peptide-clamp sites in the PA channel: the α clamp, the ϕ clamp, and the charge clamp. These clamp sites interact nonspecifically with the chemically complex translocating chain, serve to minimize unfolded state configurational entropy, and work cooperatively to promote translocation. Two models of proton gradient driven translocation have been proposed: (i) an extended-chain Brownian ratchet mechanism and (ii) a proton-driven helix-compression mechanism. These models are not mutually exclusive; instead the extended-chain Brownian ratchet likely operates on β-sheet sequences and the helix-compression mechanism likely operates on α-helical sequences. Finally, we compare and contrast anthrax toxin with other related and unrelated translocase channels.
专门的移位通道是纳米机器，它们通常（但不总是）通过膜上的狭窄孔展开并移位蛋白质。通常，这些分子机器利用外部自由能源来驱动这些反应，因为折叠的蛋白质在热力学上是稳定的，一旦展开，它们就包含巨大的扩散构象熵。为了在可感知的时间尺度上催化展开和移位未折叠状态，移位通道通常利用类似的肽夹活性位点。在这里，我们描述了炭疽毒素如何被用作生物物理模型系统来研究蛋白质移位。三部分细菌毒素由一个多聚体移位通道、保护性抗原 (PA) 和两种酶（水肿因子 (EF) 和致死因子 (LF)）组成，PA 将 EF 和 LF 移位到哺乳动物宿主细胞中。展开和移位由内体质子梯度驱动，并由 PA 通道中的三个肽夹位点催化：α 夹、ϕ 夹和电荷夹。这些夹位点与化学上复杂的移位链非特异性相互作用，有助于最小化未折叠状态的构象熵，并协同工作以促进移位。已经提出了两种质子梯度驱动移位的模型：（i）扩展链布朗棘轮机制和（ii）质子驱动螺旋压缩机制。这些模型并非相互排斥；相反，扩展链布朗棘轮机制可能在β-折叠序列上起作用，而螺旋压缩机制可能在α-螺旋序列上起作用。最后，我们将炭疽毒素与其他相关和不相关的移位通道进行了比较和对比。
