The centromere geometry essential for keeping mitosis error free is controlled by spindle forces

Abstract

This paper addresses the question of how centromere architecture affects spindle formation, and presents evidence supporting the claim that centromeres are malleable with respect to tangential forces. Once deformed, they remain in this position until they are straightened by external forces applied along microtubules. Accurate segregation of chromosomes, essential for the stability of the genome, depends on ‘bi-orientation’—simultaneous attachment of each individual chromosome to both poles of the mitotic spindle1. On bi-oriented chromosomes, kinetochores (macromolecular complexes that attach the chromosome to the spindle) reside on the opposite sides of the chromosome’s centromere2. In contrast, sister kinetochores shift towards one side of the centromere on ‘syntelic’ chromosomes that erroneously attach to one spindle pole with both sister kinetochores. Syntelic attachments often arise during spindle assembly and must be corrected to prevent chromosome loss3. It is assumed that restoration of proper centromere architecture occurs automatically owing to elastic properties of the centromere1,2. Here we test this assumption by combining laser microsurgery and chemical biology assays in cultured mammalian cells. We find that kinetochores of syntelic chromosomes remain juxtaposed on detachment from spindle microtubules. These findings reveal that correction of syntelic attachments involves an extra step that has previously been overlooked: external forces must be applied to move sister kinetochores to the opposite sides of the centromere. Furthermore, we demonstrate that the shape of the centromere is important for spindle assembly, because bipolar spindles do not form in cells lacking centrosomes when multiple chromosomes with juxtaposed kinetochores are present. Thus, proper architecture of the centromere makes an important contribution to achieving high fidelity of chromosome segregation.