Rosette synthase complex

Schematic model of the cellulose synthase complex crystalline cellulose I emerges from the rosette terminal complex, which is embedded in the plasma membrane. View from the top (above) and the side (below). (Adopted from Perez and Mazeau [42] and from Horii [40])...

This system is organized in vascular plants as a 30 nm diameter plasma membrane complex, known as the rosette TC, that was first described by Mueller and Brown more than 25 years ago [42]. The rosette is composed of an estimated 36 subunits comprising at least three types of cellulose synthase A proteins. All three components have been shown to be essential for correct assembly and enzymatic activity of the complex in Arabidopsis [43, 44]. The physical interaction of these three components has recently been demonstrated by bimolecular fluorescence complementation [45]. The system also offers a beautiful example of the functional aspects of this organization, as cortical microtubules guide the movement of cellulose synthase complexes through the plasma membrane during cell wall biosynthesis [46,47],... 

In plants, cellulose is synthesized by rosette terminal complexes (RTCs). The RTCs are hexameric protein structures containing the cellulose synthase enzyme. This enzyme synthesizes the individual cellulose chains. Each RTC is located at the interface of the cell membrane, similarly to the membrane-bound rubber transferase. Each RCT will polymerize a polysaccharide chain. This sounds a little bit complicated, but we are going to break down the synthesis into the... 

Cellulose synthesis takes place in terminal complexes (rosettes) in the plasma membrane. Each cellulose chain begins as a sitosterol dextrin formed inside the cell. It then flips to the outside, where the oligosaccharide portion is transferred to cellulose synthase in the rosette and is then extended. Each rosette produces 36 separate cellulose chains simultaneously and in parallel. The chains crystallize into one of the microfibrils that form the cell wall. 

The enzymes have several putative transmembrane domains (TMD). This is consistent with previous microscopic and biochemical data indicating that cellulose synthase is an integral membrane protein and that cellulose biosynthesis occurs at the plasma membrane (Mueller and Brown, Jr. 1980 Ross et al. 1991 Brown, Jr. et al. 1996 Delmer 1999). Visible by electron microscopy, the enzymes form large linear terminal complexes in the plasma membrane of bacteria and many algae whereas they form hexagonal rosette structures in higher plants and some algae (Mueller and Brown, Jr. 1980 Ross et al. 1991 Kimura et al. 1999). Delmer (1999) has speculated that the transmembrane domains may create a... 

Much of our understanding of the TCs has, in fact, been obtained from genetic and microscopic analyses. Interestingly, genetic analysis of cellulose-deficient mutants in plants led to the proposal that the rosette TC contains three different nonredundant cellulose synthases, and mutation in any one of these results in a defect in the assembly of cellulose microfibrils (Taylor et al. 2003). In addition, the cellulose synthases identified for cellulose synthesis in the primary cell wall are different from the cellulose synthases required for cellulose synthesis in the secondary cell wall (Robert et al. 2004). In spite of the failure to localize any other protein except the cellulose synthase to the rosette TC, other proteins have been predicted by mutant analysis to be associated with the complex and these may have a direct or indirect role in cellulose synthesis. 

It is well known that the rosette and linear terminal complexes (TCs) can be observed by the freeze-fracture replication technique. The structures revealed by this technique are known as putative cellulose-synthesizing TCs. Kimura et al. (1999) demonstrated that TCs in vascular plants contain cellulose synthases using a novel technique of sodium dodecyl sulfate (SDS)-solubilized freeze fracture replica labeling (SDS-FRL). The localization of the cellulose synthase to the TC was accomplished almost 40 years after the hypothesis of Roelofsen (1958) in which he stated that enzyme complexes could be involved in cellulose biosynthesis. It has been more than 30 years since the discovery of the first TC by Brown, Jr. and Montezinos (1976) and in particular, 26 years after the discovery of rosette TCs in plants by Mueller and Brown, Jr. (1980). 

Figure 1.3 Plant plasma membrane and ceU wall structure a) Cell waU containing cellulose microfibrils, hemicellulose, pectin, lignin and soluble proteins b) Cellulose synthase enzymes are in the form of rosette complexes, which float in the plasma membrane c) Lignification occurs in the SI, S2 and S3 layers of the cell wall. Reprinted with permission from [45, 52]. Copyright 2010 Wiley and 2008 Nature.

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