Does functional misfolding of TonB drive import across the outer membrane of Gram negative bacteria?

蛋白质具有广泛的生命必需的重要功能，包括获取和新陈代谢环境中稀缺的重要营养物质。人们普遍认为，要想发挥功能，大多数蛋白质需要折叠成明确而稳定的三维形状，即它们的结构或构象。然而，最近出现了没有固定结构的蛋白质或没有固定结构的区域，称为固有无序蛋白(IDPs)或固有无序区域(IDR)。这些蛋白质通过对环境变化的反应以及它们与大量其他蛋白质和其他复杂细胞分子相互作用的能力，在信号传递中发挥着重要作用。由于这些蛋白质没有固定的结构，对环境的微小变化也非常敏感，因此研究它们是具有挑战性的，尽管它们很重要，但仍在研究工作的确切方式。我们一直在研究一种名为TonB的蛋白质，它包含一个IDR，它被认为只是作为这个细长蛋白质两端蛋白质的两个活性部分之间的被动连接物。通过使用原子力显微镜拉伸单个蛋白质分子，我们发现在某些条件下，这种被认为是非结构的蛋白质能够抵抗拉伸(类似于绳索上的滑结)。TonB通过环境的变化在耐力的结构构象和展开的构象之间切换的能力是完全新颖的，理解这一原理可能有助于理解一个长期存在的问题，即细菌如何获得一些营养。对于一大类细菌(革兰氏阴性)来说，营养的进口是有问题的，因为它们的细胞膜外有一种叫做外膜的保护屏障。在允许它们在恶劣条件下生存的同时，外膜也起到了阻止大量营养物质进口的作用。由于外膜和内膜之间的空间(称为周质)没有能源，任何需要能量的过程，如稀缺营养的输入，都必须由内膜的能源驱动。TonB通过改变位于外膜的输入蛋白的结构来实现这种由内而外的能量传递，但其机制尚不清楚。这项研究的目的是了解横跨周质的TonB的IDR的结构和动力学的变化如何驱动输入。这一点很重要，因为从分子水平上理解这一机制既可以揭示IdP/Rs在细胞信号转导中的新功能，也可以作为设计新型抗菌剂的起点。

Proteins carry out a wide-range of important functions that are essential for life, including the acquisition and metabolism of vital nutrients that are scarce in the environment. It is well accepted that, to be functional, most proteins need to fold to well a defined and stable three dimensional shape, known as their structure or conformation. More recently, however, proteins with no fixed structure or regions of no fixed structure called intrinsically disordered proteins (IDPs) or intrinsically disordered regions (IDRs) have emerged. These proteins play an important role in signalling by both responding to changes in the environment and by their ability to interact with a multitude of other proteins and other complex cellular molecules. As these proteins don't have a fixed structure and are very sensitive to even small changes in the environment, they are challenging to study and despite their importance, the precise way in which work is still being investigated. We have been studying a protein called TonB which contains an IDR that was thought to act merely as a passive linker between the two active parts of the protein at each end of this elongated protein. By stretching single protein molecules using an atomic force microscope we found that under some conditions this supposedly unstructured protein was able to resist extension (akin to a slip knot in a rope). The ability of TonB to toggle between a force-resistant, structured conformation and an unfolded conformation by changes in environment is completely novel and understanding how this works may help to understand a long standing question of how bacteria acquire some nutrients.The import of nutrients is problematic for a large class of bacteria (Gram negative) which have a protective barrier outside their cell membrane called the outer membrane. While allowing their survival in harsh conditions, the outer membrane also acts as a barrier to the import of large nutrients. As there is no energy source in the space between outer and inner membrane (called the periplasm), any process requiring energy, such as the import of scarce nutrients, has to be driven from an energy source at the inner membrane. TonB carries out this inside-out energy transduction by altering the structure of import proteins located in the outer membrane but its mechanism is unknown. The aim of this research is to understand how changes in the structure and dynamics of the IDR of TonB that spans the periplasm drives import. This is important as a molecular-level understanding this mechanism would both reveal a novel function for IDP/Rs in cellular signalling and act as starting point for the design of novel anti-bacterial agents.