Tubules microtubules

FIGURE 17.5 The structure of an axoneme. Note the manner in which two microtubules are joined in the nine outer pairs. The smaller-diameter tubule of each pair, which is a true cylinder, is called the A-tubule and isjoined to the center sheath of the axoneme by a spoke structure. Each outer pair of tubules isjoined to adjacent pairs by a nexin bridge. The A-tubule of each outer pair possesses an outer dynein arm and an inner dynein arm. The larger-diameter tubule is known as the B-tubule. 

The membrane tubules and lamellae of the endoplasmic reticulum (ER) are extended in the cell with the use of MTs and actin filaments. Kinesin motors are required for stretching out the ER, whereas depolymerization of microtubules causes the retraction of the ER to the cell centre in an actin-dependent manner. Newly synthesized proteins in the ER are moved by dynein motors along MTs to the Golgi complex (GC), where they are modified and packaged. The resulting vesicles move along the MTs to the cell periphery transported by kinesin motors. MTs determine the shape and the position also of the GC. Their depolymerization causes the fragmentation and dispersal of the GC. Dynein motors are required to rebuild the GC. 

Allan, V.J., Vale, R.D.(1994). Movement of membrane tubules along microtubules in vitro Evidence for specialized sites of motor attachment. J. Cell Sci. (in press). 

Figure 5.2 Micrographs of metal-coated lipid tubules. Top panel shows scanning electron micrograph of copper-plated microtubules (bar = 2.0 (Jim), while bottom panel shows optical micrograph of iron-coated microtubules embedded in acrylic-urethane clear coating (bar = 25 p,m). Reprinted from Ref. 135 with permission of Wiley-VCH.

With regard to microtubular ultrastructure, micro filaments (5-7 run in diameter) are composed of filamentous actin. The tubule-like structures are formed by a, P-tubulin heterodimers. The wall is composed of 13 parallel protofilaments. Various microtubule-associated proteins and motor proteins (kinesin and dynein) are bound to the wall. The microtubule is a polar structure, i.e., plus and minus ends. 

For special purposes, tubulin may be prepared by vinblastine-induced paracrystal formation and subsequent centrifugal manipulation (Wilson et al, 1976). Dimethyl sulfoxide has been useful in promoting microtubule assembly (Himes, 1977), and deuterium oxide is also effective as a promoter of tubule assembly (Borisy et al, 1975). An example of the latter is provided by the isolation of microtubule proteins from neuroblastoma cells (Olmsted and Lyon, 1981). 

That microtubules exhibit an intrinsic polarity (i.e., with protofilaments running parallel and tubulin protomers regularly arranged in a head-to-tail manner) is obvious from experiments other than those involving image reconstruction. This intrinsic polarity was observed by Rosenbaum and Child (1976) and Witman (1975), who demonstrated biased tubulin addition to microtubules in vitro. This biased addition is a consequence of the nature of tubule-tubulin interactions at the two ends of the microtubule ... 

Small tubules Adding 6 s Elongated (250 A) proiomers microtubules... 

Double rings Adding of protomera to a Elongated Lo s of ring Elongated (480 A) template of inner ring tubules at ends microtubules... 

The term small tubules" is applied here to describe those structures having all of the characteristics of a microtubule. 

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