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

点击翻译按钮获取中文摘要

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.