The role of CESA protein modification in localisation and function of the cellulose synthase complex

纤维素是许多植物细胞壁的主要成分，被认为是世界上最丰富的天然聚合物。它是由结合在一起的长链糖葡萄糖组成的。木材是由植物的次生细胞壁组成的，特别高的比例(高达70%)的木材由纤维素组成。木材在确定农作物的机械性能方面很重要，因此在防止谷类和其他作物倒伏方面也很重要。对于工业来说，次生壁的性能直接决定了制成品的性能，例如纸张质量和用于纺织品的纤维。此外，增加可生物降解原材料比例的迫切需要可以通过在玻璃纤维等材料中使用天然植物纤维而不是合成纤维来满足。对全球变暖及其与使用汽油等化石燃料导致的碳排放上升的关联的担忧，加上全球化石燃料储量的减少，引发了人们对寻找替代燃料来源的巨大兴趣。特别令人感兴趣的是那些不会导致二氧化碳浓度增加的燃料，而且是可持续的。一个潜在的来源是使用被称为生物质的生物材料来制造“生物燃料”。最丰富的生物质来源是植物细胞壁，可以利用细胞壁中的纤维素制造乙醇或其他燃料，其方式与巴西生产的甘蔗生物燃料类似。尽管纤维素非常丰富，但与使用纤维素相关的几个技术挑战包括：(I)将其与细胞壁的其他部分分开，(Ii)打破其牢固的成键结构，以及(Iii)让植物制造更多的纤维素。令人惊讶的是，对纤维素形成过程的平等理解并不能与纤维素的重要性相提并论。纤维素是由一个非常大的酶复合体在细胞表面制造的，它的作用就像一台机器，制造许多糖链，然后这些糖链结合在一起形成纤维。纤维素合成机是不寻常的，因为它在细胞膜中移动，同时将纤维素旋转到细胞壁。这就产生了一个悖论，因为酶复合体位于膜中，膜必须足够流动，才能让复合体通过，但复合体必须结合得足够紧密，以防止酶复合体被推出膜。我们发现，酶复合体在制成IS后得到了广泛的修饰。我们想测试这种修饰针对膜上专门制造纤维素的特定区域的复合体，以及蛋白质的修饰是将复合体从细胞内移动到细胞膜的驱动力。我们将测试这是否是细胞合成纤维素能力的限制因素之一，我们将确定增加细胞修饰CESA蛋白质的能力是否会增加植物的纤维素含量，这将对制造生物燃料等应用非常有用。最后，我们想测试我们的理论，即复合体移动到细胞外，在那里CESA蛋白质修饰负责将复合体插入细胞膜的特定部分，这一过程也会导致复合体结构的巨大变化。

Cellulose is the major component of many plant cells walls and is considered to be the world's most abundant naturally occurring polymer. It is made of long chains of the sugar glucose that bind together. Wood is composed of plant secondary cell walls and a particularly high proportion (up to 70%) of wood is made up of cellulose. Wood is important in determining the mechanical properties of crop plants and consequently important in preventing cereals and other crops from falling over (lodging). For industry, the properties of the secondary walls directly determine the properties of the manufactured products, for example paper quality and the fibres used in textiles. Additionally, the pressing need to increase the proportion of our raw materials that are biodegradable can be filled by using natural plant fibres instead of synthetic fibres in materials such as fibreglass. Concerns over global warming and its links to rising carbon emissions, due to the use of fossil fuels such as petrol, coupled with diminishing worldwide fossil fuel reserves has generated huge interest in finding alternative fuel sources. Of particular interest are those fuels that do not contribute to increases in CO2 concentrations and that are sustainable. One potential source is to use biological material known as biomass to make 'biofuels'. The most abundant source of biomass is plant cell walls and it may be possible to use cellulose in the cell wall to make ethanol or other fuels in a similar manner to the sugar cane-derived biofuels produced in Brazil. Although cellulose is very abundant, there are several technological challenges associated with using cellulose including (i) separating it from other parts of the cell wall, (ii) breaking up its strongly bonded structure and (iii) getting plants to make more of it. Surprisingly, the importance of cellulose is not matched by an equal understanding of the processes behind its formation. Cellulose is made at the surface of the cell by a very large enzyme complex that acts like a machine making many chains of sugars which then bond together to form a fibre. The cellulose synthesising machine is unusual as it moves through the cell membrane whilst spinning out cellulose into the cell wall. This presents a paradox since the enzyme complex sits in a membrane that must be fluid enough to allow the complex to move through it, but the complex must be bound tightly enough to prevent the enzyme complex being pushed out of the membrane. We have found that the enzyme complex is extensively modified after is has been made. We want to test the idea that this modification targets the complex to particular regions of the membrane that are specialised for making cellulose and that it is the modification of the protein that is the driving force for moving the complex from inside the cell to the cell membrane. We will test whether this is one of the limiting factors in the cell's ability to synthesise cellulose and we will determine whether increasing the capacity of the cell to modify CESA proteins will increase the cellulose content of the plant, something that would be very useful for applications such as making biofuels. Finally, we want to test our theory which states that the complex moves to the outside of the cell where CESA protein modifications are responsible for insertion of the complex into particular parts of the cell membrane in a process that also causes a large change in the structure of the complex.