Microtubules in Xenopus oocytes, eggs, embryos, and extracts...
(Use the links at right to explore MT organization during oogenesis/development and the roles of XMAP215 and XMAP230...)
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Microtubules (MTs), composed of a- and b-tubulins (TB), play roles in a number of processes that are critical for the structure, function, and survival of eukaryotic cells. Perhaps foremost among these is their role in the assembly and function of the spindle, a complex MT-based machine that is responsible for the segregation of chromosomes during the meiotic and mitotic divisions of eukaryotes.

Tubulins are two of the most abundant proteins of Xenopus oocytes and eggs, constituting nearly 3% of the total cytoplasmic protein [1, 2]. Indeed, one fully grown oocyte or egg contains sufficient tubulin to assemble 1.5-2 kilometers of MT. Biochemical estimates suggest that 15-20% of the tubulin pool is present as polymer, corresponding to 300 meters of MT per oocyte! These results are consistent with those obtained by confocal immunofluorescence microscopy[1], which suggest that a single stage VI oocyte contains 0.5-1 million individual MTs. Several lines of evidence suggest that oocyte MTs play important roles in establishing and maintaining the animal-vegetal polarity of oocytes during stages IV-VI of oogenesis, including the transport of developmentally important maternal RNA and positioning of the germinal vesicle in the animal hemisphere (see [3], and refereces therein). MTs also play a critical role in establishment of the dorsal-ventral axis of the developing embryo ([4, 5] and references therein).


MTs are dynamic structures, undergoing alternating phases of elongation and rapid shortening, with transitions between these phases termed catastrophe and rescue [6, 7]. A third state, in which a MT neither grows nor shrinks, but remains constant in length, has been termed a “pause” [8]. This behavior, termed “dynamic instability,” has been observed both in vitro [9, 10] and in living cells; [11-13]; reviewed in [14]. However, the dynamics of individual MTs observed in vivo differs markedly from that of MTs assembled from pure subunits in vitro (see [6]).

As in other cells, the dynamics, organization, and function of MTs in Xenopus oocytes, eggs, and embryos are thought to be modulated by an array of proteins collectively referred to as microtubule-associated proteins, or MAPs (for a recent review of MAPs and their regulation, see [15]. Several MAPs with diverse roles in regulating MT dynamics and organization have been identified in Xenopus, including: (1) g-tubulin and its associated proteins in the g-TuRC ([16, 17]; reviewed in [18]); (2) OP18/stathmin, a monomer binding protein that inhibits MT assembly or induces catastrophe ([19-21]; reviewed in [22, 23]); (3) XMAP230, a putative homolog of mammalian MAP4 that promotes MT assembly and stability in a manner similar to neuronal MAPs ([5, 24-27]; reviewed in [15]); (4) end-binding proteins such as EB1, that stabilize or destabilize MTs ([28]; reviewed in [29, 30]); (5) XKCM1 [31], a member of the Kin I or kinesin-13 family of kinesins that destabilizes MTs (reviewed in [32, 33]); (6) severing proteins, such as katanin [32, 34, 35]; and (7) XMAP215, a member of the MAP215/Dis1 family that promotes MT assembly and dynamics ([36]; reviewed in [37-39]). Together, these proteins are thought to orchestrate the complex organization and function of MTs during oogenesis and early development.
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References cited:
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5. Cha, B.J. and D.L. Gard, XMAP230 is required for the organization of cortical microtubules and patterning of the dorsoventral axis in fertilized Xenopus eggs. Dev Biol, 1999. 205(2): p. 275-86.
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