Actin and myosin inhibitors block elongation of kinetochore fibre stubs in metaphase crane-fly spermatocytes

We used an ultraviolet microbeam to cut individual kinetochore spindle fibres in metaphasecrane-fly spermatocytes; then we followed the growth of the “kinetochore stubs”, the remnants of kinetochore fibres that remain attached to kinetochores. Kinetochore stubs elongate with constant velocity by adding tubulin subunits at the kinetochore, and thus elongation is related to flux of tubulin in the kinetochore microtubules. Stub elongation was blocked by cytochalasin D and latrunculin A, actin inhibitors, and by butanedione monoxime, a myosin inhibitor. We conclude that actin and myosin are involved in generating elongation and thus in producing flux of tubulin in kinetochore microtubules. We suggest that actin and myosin act in concert with a spindle matrix to propel kinetochore fibres poleward thereby causing stub elongation and generating anaphasechromosome movement in non-irradiated cells.