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General Comment |
Yang H, et al 2003 reported that MEI-1/katanin is required for translocation of the meiosis I spindle to the
oocyte cortex in C. elegans small star, filled.
In most animals, successful segregation of female meiotic chromosomes involves sequential associations of the meiosis I and meiosis II spindles with the cell cortex so that extra chromosomes can be deposited in polar bodies. The resulting reduction in chromosome number is essential to prevent the generation of polyploid embryos after fertilization. Using time-lapse imaging of living
Caenorhabditis elegans oocytes containing fluorescently labeled chromosomes or microtubules, we
have characterized the movements of meiotic spindles relative to the cell cortex. Spindle assembly
initiated several microns from the cortex. After formation of a bipolar structure, the meiosis I
spindle translocated to the cortex. When microtubules were partially depleted, translocation of the
bivalent chromosomes to the cortex was blocked without affecting cell cycle timing. In oocytes
depleted of the microtubule-severing enzyme, MEI-1, spindles moved to the cortex, but association
with the cortex was unstable. Unlike translocation of wild-type spindles, movement of
MEI-1-depleted spindles was dependent on FZY-1/CDC20, a regulator of the metaphase/anaphase
transition. We observed a microtubule and FZY-1/CDC20-dependent circular cytoplasmic
streaming in wild-type and mei-1 mutant embryos during meiosis. We propose that, in mei-1 mutant
oocytes, this cytoplasmic streaming is sufficient to drive the spindle into the cortex. Cytoplasmic
streaming is not the normal spindle translocation mechanism because translocation occurred in the
absence of cytoplasmic streaming in embryos depleted of either the orbit/CLASP homolog, CLS-2,
or FZY-1. These results indicate a direct role of microtubule severing in translocation of the meiotic
spindle to the cortex.
NCBI Summary:
Microtubules, polymers of alpha and beta tubulin subunits, form the mitotic spindle of a dividing cell and help to organize membranous organelles during interphase. Katanin is a heterodimer that consists of a 60 kDa ATPase (p60 subunit A 1) and an 80 kDa accessory protein (p80 subunit B 1). The p60 subunit acts to sever and disassemble microtubules, while the p80 subunit targets the enzyme to the centrosome. Katanin is a member of the AAA family of ATPases.
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