The cytoplasmic structural and dynamical organization of cells is due to the
presence of networks of interconnected protein polymers, referred to as the
cytoskeleton. The cytoskeleton consists of microtubules, actin microfilaments,
intermediate filaments and an organizing complex, the centrosome with its chief
component the centriole, built from two bundles of microtubules in a separated
T-shape. The centrosomal microtubules are unstable and undergo intense
dynamics. Much of the dynamic nature of microtubules is attributed to regulated
growth and shrinkage of the polymer plus ends (dynamic instability) or to the
addition of subunits at the plus end while they are simultaneously lost from the
minus end (treadmilling).
The first stage of microtubule formation is called nucleation. The process
requires tubulin, Mg 2+ and GTP and also proceeds at 370C. This stage is
relatively slow until the microtubule is initially formed. Nucleation of microtubule
assembly can occur spontaneously in a solution of purified tubulin subunits, or it
can be stimulated by addition of seeds. Several studies have examined the
pathway of early assembly, and conclude that the crucial event in nucleation is
formation and growth of a 2-D polymer, the microtubule wall. Investigation by
Erickson & Stoffler (1996) shows that third type of tubulin subunit, namely
γ-tubulin is important for nucleation. γ-tubulin forms spiral that extends a short
length of straight protofilament, which serves as a stable seed for nucleation of a
second protofilament. αφ subunits form lateral bonds to the γ-tubulin
protofilament, and longitudinal bonds to each other. When this second
protofilament has achieved three consecutive α∕β subunits, growth is more
favorable than disassembly and the microtubule should be nucleated. The
second phase, called elongation proceeds much more rapidly. The α- and
β-tubulin molecules join to form heterodimers. The heterodimers attach to other
dimers to form oligomers, which elongate to form protofilaments. Each dimer
carries two GTP molecules. However the GTP that appears to function binds to
the β-tubulin molecules. When a tubulin molecule adds to the microtubule, the
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