Abstract
Various experiments have indicated that anaphase chromosomes continue to move after their kinetochore microtubules are severed. The chromosomes move poleward at an accelerated rate after the microtubules are cut but they slow down 1–3 min later and move poleward at near the original speed. There are two published interpretations of chromosome movements with severed kinetochore microtubules. One interpretation is that dynein relocates to the severed microtubule ends and propels them poleward by pushing against non-kinetochore microtubules. The other interpretation is that components of a putative “spindle matrix” normally push kinetochore microtubules poleward and continue to do so after the microtubules are severed from the pole. In this study we distinguish between these interpretations by treating cells with taxol. Taxol eliminates microtubule dynamics, alters spindle microtubule arrangements, and inhibits dynein motor activity in vivo. If the dynein interpretation is correct, taxol should interfere with chromosome movements after kinetochore microtubules are severed because it alters the arrangements of spindle microtubules and because it blocks dynein activity. If the “spindle matrix” interpretation is correct, on the other hand, taxol should not interfere with the accelerated movements. Our results support the spindle matrix interpretation: anaphase chromosomes in taxol-treated crane-fly spermatocytes accelerated after their kinetochore microtubules were severed.
Highlights
Experiments in this report deal with movements of anaphase chromosomes in crane-fly spermatocytes when their kinetochore microtubules are severed
Treatment of entire crane-fly testes with 5–10 μM taxol causes spermatocyte spindles to change shape and spindle microtubules to rearrange but does not block anaphase onset (LaFountain et al, 2001); anaphase movements are slowed considerably, from average speeds of 0.5 μm/min in control cells to average speeds of 0.1 μm/min in taxol-treated cells (LaFountain et al, 2001). In those experiments LaFountain et al (2001) showed that taxol interferes with normal spindle transport mechanisms: in control cells, akinetic arm fragments produced in metaphase move poleward at the same speeds as anaphase chromosomes, but in taxol-treated cells the arm fragments do not move at all
In our experiments nanomolar concentrations of taxol were added directly to crane-fly spermatocytes and we monitored whether chromosomes accelerated after their kinetochore fibers were cut with a laser microbeam
Summary
Experiments in this report deal with movements of anaphase chromosomes in crane-fly spermatocytes when their kinetochore microtubules are severed. Treatment of entire crane-fly testes with 5–10 μM taxol causes spermatocyte spindles to change shape and spindle microtubules to rearrange but does not block anaphase onset (LaFountain et al, 2001); anaphase movements are slowed considerably, from average speeds of 0.5 μm/min in control cells to average speeds of 0.1 μm/min in taxol-treated cells (LaFountain et al, 2001) In those experiments LaFountain et al (2001) showed that taxol interferes with normal spindle transport mechanisms: in control cells, akinetic arm fragments produced in metaphase (by severing arms) move poleward at the same speeds as anaphase chromosomes, but in taxol-treated cells the arm fragments do not move at all. In our experiments nanomolar concentrations of taxol were added directly to crane-fly spermatocytes and we monitored whether chromosomes accelerated after their kinetochore fibers were cut with a laser microbeam
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