Cytoplasmic microtubular system implicated in de novo formation of a Rabl-like orientation of chromosomes in fission yeast

Chromosomes are not packed randomly in the nucleus. The Rab1 orientation is an example of the non-random arrangement of chromosomes, centromeres are g rouped in a limited area near the nuclear periphery and telomeres are locat ed apart from centromeres. This orientation is established during mitosis a nd maintained through subsequent interphase in a range of species. We repor t that a Rab1-like configuration can be formed de novo without a preceding mitosis during the transition from the sexual phase to the vegetative phase of the life cycle in fission yeast. In this process, each of the dispersed centromeres is often associated with a novel Sad1-containing body that is contacting a cytoplasmic microtubule laterally (Sad1 is a component of the spindle pole body (SPB)). The Sad1-containing body was colocalized with oth er known SPB components, Kms1 and Spo15 but not with Cut12, indicating that it represents a novel SPB-related complex. The existence of the triplex st ructure (centromere-microtubule-Sad1 body) suggests that the clustering of centromeres is controlled by a cytoplasmic microtubular system. Accordingly , when microtubules are destabilized, clustering is markedly reduced.