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

蛋白质执行对生命至关重要的广泛重要功能，包括获取和代谢环境中稀缺的重要营养素。众所周知，为了发挥功能，大多数蛋白质需要折叠成明确且稳定的三维形状，称为其结构或构象。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，称为本质无序蛋白质（IDP）或本质无序区域（IDR）。这些蛋白质通过响应环境的变化以及它们与许多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导中发挥重要作用。由于这些蛋白质没有固定的结构，并且对环境中的微小变化非常敏感，因此研究它们具有挑战性，尽管它们很重要，但工作的精确方式仍在研究中。我们一直在研究一种名为TonB的蛋白质，它含有一个IDR，该IDR被认为只是作为该蛋白质两端两个活性部分之间的被动接头。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，这种所谓的非结构化蛋白质能够抵抗拉伸（类似于绳子上的滑结）。TonB能够通过环境的变化在抗力、结构化构象和未折叠构象之间切换，这是一种全新的能力，了解其工作原理可能有助于理解细菌如何获得某些营养素这一长期存在的问题。对于一大类细菌（革兰氏阴性菌）来说，营养素的输入是有问题的，这些细菌在其细胞膜外有一个保护屏障，称为外膜。在允许它们在恶劣条件下生存的同时，外膜也是大量营养物质输入的屏障。由于外膜和内膜之间的空间（称为周质）没有能量来源，因此任何需要能量的过程，例如稀有营养素的输入，都必须由内膜的能量来源驱动。TonB通过改变位于外膜的输入蛋白的结构来进行这种由内而外的能量转导，但其机制尚不清楚。本研究的目的是了解跨越周质的TonB IDR的结构和动力学变化如何驱动输入。这是重要的，因为分子水平的理解这种机制将揭示IDP/Rs在细胞信号传导中的新功能，并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.