In cell biology, the spindle apparatus is the cytoskeletal structure of eukaryotic cells that forms during cell division to separate sister chromatids between daughter cells. It is referred to as the mitotic spindle during mitosis, a process that produces genetically identical daughter cells, or the meiotic spindle during meiosis, a process that produces gametes with half the number of chromosomes of the parent cell. Besides chromosomes, the spindle apparatus is composed of hundreds of proteins. Microtubules comprise the most abundant components of the machinery.
Spindle structure
Attachment of microtubules to chromosomes is mediated by kinetochores, which actively monitor spindle formation and prevent premature anaphase onset. Microtubule polymerization and depolymerization dynamics drive chromosome congression. Depolymerization of microtubules generates tension at kinetochores; bipolar attachment of sister kinetochores to microtubules emanating from opposite cell poles couples opposing tension forces, aligning chromosomes at the cell equator and poising them for segregation to daughter cells. Once every chromosome is bi-oriented, anaphase commences and cohesin, which couples sister chromatids, is severed, permitting the transit of the sister chromatids to opposite poles. The cellular spindle apparatus includes the spindle microtubules, associated proteins, which include kinesin and dynein molecular motors, condensed chromosomes, and any centrosomes or asters that may be present at the spindle poles depending on the cell type. The spindle apparatus is vaguely ellipsoid in cross section and tapers at the ends. In the wide middle portion, known as the spindle midzone, antiparallel microtubules are bundled by kinesins. At the pointed ends, known as spindle poles, microtubules are nucleated by the centrosomes in most animal cells. Acentrosomal or anastral spindles lack centrosomes or asters at the spindle poles, respectively, and occur for example during female meiosis in most animals. In this instance, a Ran GTP gradient is the main regulator of spindle microtubule organization and assembly. In fungi, spindles form between spindle pole bodies embedded in the nuclear envelope, which does not break down during mitosis.
Microtubule-associated proteins and spindle dynamics 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. 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. 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. 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.
Organizing the spindle apparatus
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