The molecular basis for ribophorin I function during N-glycosylation

构成构成我们身体的细胞的关键组成部分的蛋白质通常通过连接糖或形成所谓的糖蛋白的多聚糖来修饰。糖蛋白的一个常见而重要的类别是N-连接的种类，其中许多分子是生命所必需的，在与发育和健康相关的重要作用中发挥着重要作用。寡糖转移酶(OST)复合体是一种生物机器，当蛋白质在细胞内合成时，它会将N-连接的糖链添加到蛋白质中。虽然高等生物的寡糖转移酶是一个具有许多不同成分或亚基的大型酶复合体，但细菌的酶要简单得多，只有一个亚基似乎具有类似的作用。这样的比较让我们对人类寡糖转移酶机制的各种额外亚基的功能产生了疑问，特别是询问了一个特定的额外亚基，称为核糖蛋白I，可能如何改变或改善糖蛋白的产生。我们最近发现核糖蛋白I是哺乳动物寡糖转移酶的一个非催化亚基。实际上，这意味着虽然核糖蛋白I对于某些蛋白质的N-糖基化是必不可少的，但其他蛋白质可能根本不需要核糖蛋白I。我们提出了一个模型，其中寡糖转移酶复合体有一个名为STT3蛋白的中央机器，并认为正是这个成分做了实际的N-糖基化‘工作’。核糖体蛋白I的作用是通过某种方式帮助不寻常或困难的蛋白质正确地传递到STT3蛋白，并以一种允许它们被有效糖基化的方式来帮助这一中央机器。这个项目的目的是调查核糖体蛋白I到底做了什么，并确定它是如何作为这台更大的机器的一部分发挥作用的。为此，我们将回答两个基本问题：第一，蛋白质的什么性质意味着它需要核糖蛋白I的帮助才能被适当的N-糖基化；第二，核糖蛋白I实际上如何辅助核心STT3亚单位，从而使这些蛋白质能够被寡糖转移酶复合体有效地处理？

The proteins that form the key building blocks of the cells that make up our bodies are often modified by the attachment of sugars or glycans forming so called glycoproteins. One common and important class of glycoprotein is the 'N-linked' variety, and many of these molecules are essential for life playing important roles related to development and good health. The oligosaccharyltransferase (OST) complex is the biological machine that adds N-linked glycans to proteins whilst they are being made inside the cell. Whilst the oligosaccharyltransferase of higher organisms is a large enzyme complex with a number of different components or subunits, bacteria have a much simpler enzyme with only one subunit that appears to perform a similar role. Such comparisons have led us to question the function of the various extra subunits of the human oligosaccharyltransferase machinery, and in particular to ask how one particular additional subunit, called ribophorin I, might act to alter or improve the production of glycoproteins. We recently discovered that ribophorin I acts as a non-catalytic subunit of the mammalian oligosaccharyltransferase. In practice this means that whilst ribophorin I is essential for the N-glycosylation of some proteins, may others do not need ribophorin I at all. We propose a model where the oligosaccharyltransferase complex has a central machine called the STT3 protein, and suggest that it is this component that does the actual 'work' of N-glycosylation. The role of ribophorin I would then be to assist this central machine by somehow helping unusual or difficult proteins to be delivered to the STT3 protein correctly and in a way that allows them to be efficiently glycosylated. The aim of this project is to investigate exactly what ribophorin I does and to establish how it carries out its role as part of this larger machine. To do this we will answer two fundamental questions: firstly, what are the properties of a protein that mean it needs the help of ribophorin I in order to be properly N-glycosylated; and secondly, how does ribophorin I actually assist the core STT3 subunit so that these proteins can be dealt with efficiently by the oligosaccharyltransferase complex?