Chromatin-induced spindle assembly plays an important role in metaphase congression of silkworm holocentric chromosomes

Abstract

The kinetochore plays important roles in cell cycle progression. Interactions between chromosomes and spindle microtubules allow chromosomes to congress to the middle of the cell and to segregate the sister chromatids into daughter cells in mitosis. The chromosome passenger complex (CPC), composed of the Aurora B kinase and its regulatory subunits INCENP, Survivin, and Borealin, plays multiple roles in these chromosomal events. In the genome of the silkworm, Bombyx mori, which has holocentric chromosomes, the CPC components and their molecular interactions were highly conserved. In contrast to monocentric species, however, the silkworm CPC co-localized with the chromatin-driven spindles on the upper side of prometaphase chromosomes without forming bipolar mitotic spindles. Depletion of the CPC by RNAi arrested the cell cycle progression at prometaphase and disrupted the microtubule network of the chromatin-driven spindles. Interestingly, depletion of mitotic centromere-associated kinesin (MCAK) recovered formation of the microtubule network but did not overcome the cell cycle arrest at prometaphase. These results suggest that the CPC modulates the chromatin-induced spindle assembly and metaphase congression of silkworm holocentric chromosomes.