Molecular and functional characterisation of unanchored polyubiquitin

蛋白质是所有生物体中最丰富的分子，生命过程依赖于它们与其他蛋白质的受控相互作用。蛋白质之间的相互作用可以通过一种称为翻译后修饰的过程来开启。在这种情况下，第一个蛋白质通过将另一个分子附着在其表面来修饰，然后这个附加的修饰被与之结合的第二个蛋白质识别；因此，两种蛋白质可以相互作用。细胞利用这一过程作为一种信号，表明一种特定的蛋白质已经耗尽或损坏，需要被摧毁。需要处理的蛋白质被另一种叫做泛素的分子修饰，一种叫做蛋白酶体的特殊蛋白质，它本身就是一种蛋白酶（一种能够消化其他蛋白质的蛋白质），识别出这种泛素修饰的目标并将其清除。然而，泛素修饰并不总是表明靶标的处理；有时，这种修饰仅仅是为了使两种蛋白质靠得更近，这样它们就可以一起发挥作用。事实上，在每种情况下，通常会有几个泛素修饰因子的拷贝以一种称为“多泛素链”的结构附着在目标蛋白上。最近一篇非常重要的科学论文证明，细胞中存在“非锚定”形式的多泛素链——它们相当于在其他蛋白质上发现的修饰，但实际上并不附着在目标上——它们本身具有非常特定的功能。然而，到目前为止，还没有人纯化过这些无锚定的多泛素链，并精确分析过它们的组成。我们已经开发了一种全新的方法，允许首次纯化无锚定的多泛素链。我们打算从不同的生物样本中纯化这些分子，并找出它们含有什么。我们还将研究当细胞开启它们的蛋白质处理系统时，以及当它们对来自细胞外的信号或信息作出反应时，未锚定的多泛素链会发生什么。通过研究其他蛋白质控制非锚定多泛素链的形成或去除，以及不同的非锚定多泛素链实际起什么作用，我们将更清楚地了解泛素如何控制细胞的一些绝对基本功能。了解这些信息非常重要，因为尽管细胞中的“正常”过程是由泛素控制的，但许多人类疾病实际上是由这些过程中的缺陷引起的。例如，在阿尔茨海默氏症和帕金森症等神经退行性疾病中，通常由泛素控制的蛋白质处理系统不能正常工作。同样，在一些骨病中，使用泛素对细胞外信号作出反应的细胞系统也有缺陷。只有正确理解泛素分子通常控制的过程，我们才能开始开发真正有效的治疗方法。

Proteins are the most abundant molecules in all living organisms and the life process relies on their controlled interactions with other proteins. One way in which interactions between proteins can be switched on is by a process known a post-translational modification. In this case, a first protein is modified by attaching another molecule on to its surface, and this appended modification is then recognised by a second protein which binds to it; thus, two proteins can be made to interact. Cells use this process as a way of signalling that a particular protein is worn out or damaged and needs to be destroyed. The protein that needs to be disposed of becomes modified by another molecule called ubiquitin, and a specialised protein called the proteasome, which itself is a protease (a protein that is able to digest other proteins) recognises this ubiquitin-modified target and removes it. Modification with ubiquitin does not however always signal disposal of the target; sometimes the modification simply serves to bring two proteins in to close proximity so they can function together. In each case, in fact usually several copies of the ubiquitin modifier become attached on to the target protein in a structure known as a 'polyubiquitin chain'. A very important recent scientific paper has demonstrated that 'unanchored' forms of polyubiquitin chains exist in cells - these are equivalent to the modifications found on other proteins, but are not actually attached to targets - and they themselves have very specific functions. However, to date no one has ever purified these unanchored polyubiquitin chains and analysed precisely what they are composed of. We have developed a completely new method that allows for the first time unanchored polyubiquitin chains to be purified. We intend to purify these molecules from different biological samples, and work out what they contain. We will also investigate what happens to unanchored polyubiquitin chains when cells switch on their protein disposal systems, and when they respond to signals or messages from outside the cell. By investigating what other proteins control the formation or removal of unanchored polyubiquitin chains, and what jobs different unanchored polyubiquitin chains actually do, we will have a much clearer picture of how ubiquitin controls some of the absolutely essential functions of the cell. It is really important to know this information, because although 'normal' processes in the cell are controlled by ubiquitin, many human diseases are actually caused by defects in these processes. For example, in neurodegenerative diseases such as Alzheimer's and Parkinson's, the protein disposal system which ubiquitin normally controls does not function properly. Likewise, in some bone diseases the cellular systems which use ubiquitin to respond to signals from outside the cell are defective. We can only start to develop really effective treatments for these conditions with a proper understanding of the processes that the ubiquitin molecule normally controls.