Microtubules structures, special

The interior of the living cell is occupied by structural elements such as microtubules and filaments, organelles, and a variety of other macromolecular species making it an environment with special characteristics [97], These systems are crowded since collectively the macromolecular species occupy a large volume fraction of the cell [98, 99]. Crowding can influence both the... 

Abstract Tubulin is a fascinating molecule that forms the cytoskeleton of the cells and plays an important role in cell division and trafficking of molecules. It polymerizes and depolymerizes in order to fulfill this biological function. This function can be modulated by small molecules that interfere with the polymerization or the depolymerization. In this article, the structural basis of this behavior is reviewed with special attention to the contribution of NMR spectroscopy. Complex structures of small molecules that bind to tubulin and microtubules will be discussed. Many of them have been determined using NMR spectroscopy, which proves to be an important method in tubulin research. 

Three types of microtubule can readily be defined in the mitotic spindle. Polar microtubules overlap (and probably interact) between the poles and are involved in pushing the poles apart in anaphase. Astral microtubules radiate in all directions and also help separate the poles. Kinetochore microtubules attach themselves to specialized protein structures (kinetochores) located on each side of the centromere of each chromosome. These microtubules are involved in moving the chromosomes to the metaphase plate and in separating sister chromatids at anaphase. The microtubules in the spindle are very dynamic and have a half-life of only a few seconds. This appears to be especially important in the capture of chromosomes by the kinetochore microtubules. Microtubules that miss the target kinetochores are quickly lost because their dynamic instability soon leads to depolymerization. The new microtubules that form may hit the target and be partially stabilized through plus-end capping. 

Virtually every microtubule in a cell is a simple tube, a singlet microtubule, built from 13 protofilaments. In rare cases, singlet microtubules contain more or fewer protoflla-ments for example, certain microtubules in the neurons of nematode worms contain 11 or 15 protofilaments. In addition to the simple singlet structure, doublet or triplet microtubules are found in specialized structures such as cilia and flagella (doublet microtubules) and centrioles and basal bodies (triplet microtubules). Each doublet or triplet contains one complete 13-protofllament microtubule (A tubule) and one or two additional tubules (B and C) consisting of 10 protofilaments (Figure 20-4). 

Spindle fine structure details of special significance for functional interpretations are the exact numbers of each class of microtubules in particular spindles, their lengths... 

The spatial orientation of cell wall constituents arises from inhomogeneities of the plasma membrane. These may be brought about by the fusion of Golgi vesicles or ER with the membrane at distinct sites. The site of fusion seems to be directed by microtubules and other cytoskeletal structures, as well as by the phenomenon of membrane recognition, i.e., the special affinity of the vesicles to certain kinds and areas of membranes. 

Several attempts were made to separate the mitotic apparatus, that is, the spindle, the matrix substance found between the fibers, and possibly other chemicals directly associated with the fibers or the matrix, from the remaining nuclear structure. Most of these studies have yielded results that are difficult to interpret. Only recently has it become obvious that the fibers are made of special proteins that constitute the microtubules [242-246]. 

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