Physics Department, Trento University, Via Sommarive 14, Povo (Trento) 38123, Italy.
Biomolecules. 2013 Nov 18;3(4):967-85. doi: 10.3390/biom3040967.
Euplotes nobilii and Euplotes raikovi are phylogenetically closely allied species of marine ciliates, living in polar and temperate waters, respectively. Their evolutional relation and the sharply different temperatures of their natural environments make them ideal organisms to investigate thermal-adaptation. We perform a comparative study of the thermal unfolding of disulfide-rich protein pheromones produced by these ciliates. Recent circular dichroism (CD) measurements have shown that the two psychrophilic (E. nobilii) and mesophilic (E. raikovi) protein families are characterized by very different melting temperatures, despite their close structural homology. The enhanced thermal stability of the E. raikovi pheromones is realized notwithstanding the fact that these proteins form, as a rule, a smaller number of disulfide bonds. We perform Monte Carlo (MC) simulations in a structure-based coarse-grained (CG) model to show that the higher stability of the E. raikovi pheromones is due to the lower locality of the disulfide bonds, which yields a lower entropy increase in the unfolding process. Our study suggests that the higher stability of the mesophilic E. raikovi phermones is not mainly due to the presence of a strongly hydrophobic core, as it was proposed in the literature. In addition, we argue that the molecular adaptation of these ciliates may have occurred from cold to warm, and not from warm to cold. To provide a testable prediction, we identify a point-mutation of an E. nobilii pheromone that should lead to an unfolding temperature typical of that of E. raikovi pheromones.
高贵游仆虫和瑞氏游仆虫是亲缘关系密切的海洋纤毛虫，分别生活在极地和温带水域。它们的进化关系和自然环境中截然不同的温度使它们成为研究热适应的理想生物。我们对这两种纤毛虫产生的富含二硫键的信息素的热变性进行了比较研究。最近的圆二色性（CD）测量表明，尽管这两种亲嗜冷（E. nobilii）和中温（E. raikovi）蛋白家族具有非常接近的结构同源性，但它们的变性温度却有很大的不同。尽管 E. raikovi 信息素形成的二硫键数量通常较少，但这些蛋白质具有增强的热稳定性。我们在基于结构的粗粒化（CG）模型中进行蒙特卡罗（MC）模拟，结果表明 E. raikovi 信息素的稳定性更高是由于二硫键的局部性较低，从而在变性过程中增加的熵较小。我们的研究表明，中温的 E. raikovi 信息素的稳定性不是主要由于存在强烈的疏水性核心，正如文献中提出的那样。此外，我们认为这些纤毛虫的分子适应可能是从冷到暖，而不是从暖到冷。为了提供一个可测试的预测，我们确定了一个 E. nobilii 信息素的点突变，它应该导致典型的 E. raikovi 信息素的变性温度。
