Interactions between checkpoint proteins the kinetochore. Before pairs of sister chromatids (shown in blue on the right) can be pulled apart during cell division, structures called spindle assembly checkpoints (SACs) form on the kinetochores (red circles) of each sister chromatid. The domain architecture of an important kinetochore protein called KNL1 is shown for four species, together with the name of the protein in that species and the number of amino acids it contains: S. cerevisiae (budding yeast, top); S. pombe (fission yeast); C. elegans (worm); Human (bottom). In experiments on budding yeast Primorac et al. have shown that the checkpoint protein Bub3 (green) binds to MELT motifs (red) that have been phosphorylated (P) by the enzyme Mps1, and that the checkpoint protein Bub1 (gold) then binds to Bub3 (and lies in almost the same plane as Bub3). A portion of the crystal structure displaying the interaction between Bub3, Bub1 and the phosphorylated MELT peptide (magenta) is also shown. The human version of KNL1 is the only version to have KI motifs (grey, see text); PP1-binding sites (blue) and coiled-coil kinetochore-binding domains (dark green) are also shown.