Rosette structure

Significantly, the ordered pattern of fibril deposition in the secondary wall of Micrasterias367 was shown to be derived from the structure of the complexes located in the plasma membrane. The results indicated that the widest fibrils, those in the center of a band, are formed by the longest rows of rosettes, those in the center of arosette array. The shorter rows of rosettes within an array give rise to narrower fibrils. This proportionality between the width of a secondary cellulose fibril and the number of rosettes involved in its formation provides strong evidence that the rosette structure plays a significant role in the synthesis of cellulose fibrils. 

Similar rosette-structures have been detected on the P-fracture faces of membranes of cytoplasmic vesicles in Micrasterias.367 The freeze-fracture images obtained suggested that the rosette complexes are first assembled into cytoplasmic vesicles, and are subsequently incorporated into the plasma membrane by a process of vesicle fusion. This supports the evidence obtained by Kiermayer and Dobberstein,371,372 in Micrasterias, that the flat vesicles that contain the rosette complexes are formed by the Golgi apparatus. 

Fig. 7.5 Self-assembled CC units forming a complete rosette structure.

Nagore, N., Howe, S., Boxer, L., Scheuer, P.J. Liver cell rosettes structural differences in cholestasis and hepatitis. Liver 1989 9 43-51... 

Figure 6. Whether a particular molecular complex adopts the tape or rosette structure depends on (what appear to be) relatively minor modifications in the molecular structures of the starting materials.

Figure 6 (opposite) Melamine-barbituric acid recognition pattern and a schematic depiction of the linear tape, crinkled tape and rosette structures derived from this pattern (see Zerkowski et al. [68].)... 

The simplest design is a unimolecular ion channel. Nevertheless, designing and synthesizing such macromolecules is complicated, and numerous examples of ion channels rely in the self-assembly of several structures within the membrane to form the functional pore. Depending on the type of molecules macrocycles, staves, or even smaller molecules can form barrel-hoop, barrel staves, or barrel rosette structures (Figure 7). 

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... 

Studies on mutants affecting cellulose synthesis in the primary cell wall have also led to the idea that three catalytic subunits are required for cellulose synthesis in these cell types. These genes are different from those required in the secondary cell wall (Arioli et al. 1998 Fagard et al. 2000 Scheible et al. 2001). Work on the rswl mutant of Arabidopsis demonstrates that in addition to their catalytic fimc-tion, CesA proteins are also essential for determining rosette structure, rswl-1 is caused by a comparatively small amino acid change (Ala to Val) in AtCesAl, yet at the restrictive temperature the hexameric rosettes are replaced by smaller structures (Arioli et al. 1998). 

Figure 2. Schematic representations of preorganization of three melamine by covalent attachment to a center hub (left) and the formation of a rosette structure due to large groups (depicted here as spheres) by peripheral crowding (right). Melamines and cyanuric acid are represented as disks.

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