<P> The dynamic lengthening and shortening of spindle microtubules, through a process known as dynamic instability determines to a large extent the shape of the mitotic spindle and promotes the proper alignment of chromosomes at the spindle midzone. Microtubule - associated proteins (MAPs) associate with microtubules at the midzone and the spindle poles to regulate their dynamics . γ - tubulin is a specialized tubulin variant that assembles into a ring complex called γ - TuRC which nucleates polymerization of α / β tubulin heterodimers into microtubules . Recruitment of γ - TuRC to the pericentrosomal region stabilizes microtubule minus - ends and anchors them near the microtubule - organizing center. The microtubule - associated protein Augmin acts in conjunction with γ - TURC to nucleate new microtubules off of existing microtubules . </P> <P> The growing ends of microtubules are protected against catastrophe by the action of plus - end microtubule tracking proteins (+ TIPs) to promote their association with kinetochores at the midzone . CLIP170 was shown to localize near microtubule plus - ends in HeLa cells and to accumulate in kinetochores during prometaphase . Although how CLIP170 recognizes plus - ends remains unclear, it has been shown that its homologues protect against catastrophe and promote rescue, suggesting a role for CLIP170 in stabilizing plus - ends and possibly mediating their direct attachment to kinetochores . CLIP - associated proteins like CLASP1 in humans have also been shown to localize to plus - ends and the outer kinetochore as well as to modulate the dynamics of kinetochore microtubules (Maiato 2003). CLASP homologues in Drosophila, Xenopus, and yeast are required for proper spindle assembly; in mammals, CLASP1 and CLASP2 both contribute to proper spindle assembly and microtubule dynamics in anaphase . Plus - end polymerization may be further moderated by the EB1 protein, which directly binds the growing ends of microtubules and coordinates the binding of other + TIPs . </P> <P> Opposing the action of these microtubule - stabilizing proteins are a number of microtubule - depolymerizing factors which permit the dynamic remodeling of the mitotic spindle to promote chromosome congression and attainment of bipolarity . The kinesin - 13 superfamily of MAPs contains a class of plus - end - directed motor proteins with associated microtubule depolymerization activity including the well - studied mammalian MCAK and Xenopus XKCM1 . MCAK localizes to the growing tips of microtubules at kinetochores where it can trigger catastrophe in direct competition with stabilizing + TIP activity . These proteins harness the energy of ATP hydrolysis to induce destabilizing conformational changes in protofilament structure that cause kinesin release and microtubule depolymerization . Loss of their activity results in numerous mitotic defects . Additional microtubule destabilizing proteins include Op18 / stathmin and katanin which have roles in remodeling the mitotic spindle as well as promoting chromosome segregation during anaphase . </P> <P> The activities of these MAPs are carefully regulated to maintain proper microtubule dynamics during spindle assembly, with many of these proteins serving as Aurora and Polo - like kinase substrates . </P>

What is a spindle what is its function