Microtubule polymerization- depolymerization equilibrium

The mitotic spindle is part of a larger intracellular skeleton (cytoskele-ton) that is essential for the internal movements occurring in the cytoplasm of all eukaryotic cells. The mitotic spindle consists of chromatin, and a system of microtubules composed of the protein tubulin. The mitotic spindle is essential for the equal partitioning of DNA into the two daughter cells formed when a eukaryotic cell divides. Several plant-derived substances used as anticancer drugs disrupt this process by affecting the equilibrium between the polymerized and depolymerized forms of the microtubules, thereby causing cytotoxicity. 

The force which propels secretory granules along the microtubules is less clear. It is known that the micro tubular system exists in at least two states the fully polymerized form represented by intact microtubules, and the disintegrated form represented by a pool of depolymer-ized globular proteins (tubulin) in the cytoplasm. In order for microtubules to function properly, a dynamic state of equilibrium must exist between the fully-formed tubules and the tubule constituent pool. Thus, colchicine and other antimitotic agents bind to specific sites on the microtubular subunits. It has been proposed that they exert their effect by inactivating the free subunits and thereby shift the equilibrium between the associated and dissociated states of the microtubules so that eventually no intact microtubules remain and secretion is inhibited. Similarly, stabilization of microtubules in the polymerized form with D2O also inhibits cellular secretion of insulin. From this, one can hypothesize that if the secretory vesicles were somehow attached to the microtubules, possibly by way of microfilaments, a constant cycle of depolymerization near the cell periphery, with a repolymerization at the central area of the cell, would advance the secretory vesicle from the cell center to the cell web. In addition, if tubulin actually contains an actin-like contractile protein, then this contractile property may well contribute to the intracellular movement of secretory materials. 

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