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。它是由葡萄糖的长链结合在一起。木材由植物次生细胞壁组成，特别高比例（高达70%）的木材由纤维素组成。木材在确定作物的机械性能方面很重要，因此在防止谷物和其他作物倒伏（倒伏）方面也很重要。对于工业而言，次级壁的性能直接决定了所制造产品的性能，例如纸张质量和纺织品中使用的纤维。此外，我们迫切需要增加可生物降解的原材料比例，这可以通过在材料中使用天然植物纤维而不是合成纤维来满足，例如人造玻璃。对全球变暖及其与由于使用诸如汽油的化石燃料而导致的碳排放增加的联系的担忧，加上世界范围内化石燃料储量的减少，已经产生了对寻找替代燃料源的巨大兴趣。特别令人感兴趣的是那些不会导致二氧化碳浓度增加并且是可持续的燃料。一个潜在的来源是使用生物质来制造“生物燃料”。最丰富的生物质来源是植物细胞壁，并且可以使用细胞壁中的纤维素以与巴西生产的甘蔗衍生生物燃料类似的方式制造乙醇或其他燃料。虽然纤维素非常丰富，但使用纤维素存在一些技术挑战，包括（i）将其与细胞壁的其他部分分离，（ii）打破其牢固的结合结构，以及（iii）让植物产生更多的纤维素。令人惊讶的是，纤维素的重要性与对其形成背后的过程的同等理解并不匹配。纤维素是在细胞表面由一个非常大的酶复合体制造的，它的作用就像一台机器，制造许多糖链，然后将糖链结合在一起形成纤维。纤维素合成机器是不寻常的，因为它穿过细胞膜，同时将纤维素纺出细胞壁。这提出了一个悖论，因为酶复合物位于膜中，该膜必须具有足够的流动性以允许复合物穿过它，但复合物必须足够紧密地结合以防止酶复合物被推出膜。我们发现，酶复合物在制成后被广泛地修饰。我们想测试这样一种想法，即这种修饰将复合物靶向膜的特定区域，这些区域专门用于制造纤维素，并且蛋白质的修饰是将复合物从细胞内部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，并确定增加细胞修饰CESA蛋白的能力是否会增加植物的纤维素含量，这对于制造生物燃料等应用非常有用。最后，我们想要测试我们的理论，该理论指出复合物移动到细胞外部，其中CESA蛋白质修饰负责将复合物插入细胞膜的特定部分，该过程也导致复合物结构的巨大变化。

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.