Functional characterization of cohesin SMC3 and separase and their roles in the segregation of large and minichromosomes in Trypanosoma brucei

Minichromosomes in the nuclear genome of Trypanosoma brucei exhibit unusual patterns of mitotic segregation. To address whether differences in their mode of segregation in relation to large chromosomes are reflected at a molecular level, we characterized two different proteins that have highly conserved functions in eukaryotic chromosomes segregation: the SMC3 protein, a component of the chromatid cohesion apparatus, and the protease separase that resolves the cohesin complex at the onset of anaphase and has, in other organisms, additional functions during mitosis. Using in situ hybridization we show that RNA interference-mediated depletion of SMC3 has no visible effect on the segregation of the minichromosomal population but interferes with the faithful mitotic separation of large chromosomes. In contrast, separase depletion causes missegregation of both mini- and large chromosomes. We also show that SMC3 persists as a soluble protein throughout the cell cycle and only associates with chromatin between G1 and metaphase. Separase is present in the cell during the entire cell cycle, but is excluded from the nucleus until the metaphase–anaphase transition, thereby providing a potential control mechanism to prevent the untimely cleavage of the cohesin complex.