Sgo1 establishes the centromeric cohesion protection mechanism in G2 before subsequent Bub1-dependent recruitment in mitosis.

Bub1 was one of the first protein kinases identified as a component of the spindle-assembly checkpoint, a surveillance mechanism that delays anaphase onset until all chromosomes are stably attached to spindle microtubules. Whereas the kinase activity of Bub1 is not required for checkpoint function in yeast, its requirement in mammalian cells is still unclear. Using a complementation assay with bona fide BUB1-null mouse embryonic fibroblasts, we show that the kinase activity of Bub1 is not required for checkpoint function or chromosome alignment. Its activity is, however, required for centromeric localisation of Sgo1, a known protector of centromeric cohesion. Despite the absence of Sgo1 from mitotic centromeres in cells devoid of Bub1 activity, centromeric cohesion is still maintained until anaphase. An explanation for this comes from observations showing that Sgo1 is first recruited to centromeric heterochromatin in G2, but then becomes diffusely localised throughout the nucleus in early prophase, before returning to centromeres later in prophase. Importantly, whereas centromeric localisation of Sgo1 in prophase is dependent on the kinase activity of Bub1, its recruitment to centromeric heterochromatin in G2 is not. Rather, the localisation of Sgo1 in G2 is abolished when heterochromatin protein 1 is not bound to centromeric heterochromatin. Thus, it seems that Sgo1 sets up the centromeric protection mechanism in G2, but that its Bub1-dependent localisation to centromeres during mitosis is not required to maintain cohesion.