Fig. 3.

Mechanisms for SAC protein recruitment at kinetochores. (A) The Bub1-Bub3-BubR1-Cdc20 module docks onto repetitive motifs in the Knl1 N-terminus that are phosphorylated by the kinase Mps1, which is recruited to kinetochores by the Ndc80 complex; Plk1 kinase activity targeting Knl1 also contributes to a varying extent, depending on the species. (B) Localization of the Mad1-Mad2 complex to unattached kinetochores is mediated by Bub1, the RZZ complex and the Ndc80-Mps1 module. Mps1 phosphorylates Bub1 CM1 to promote its interaction with Mad1 and may also target the RZZ complex; the Ndc80 complex may also contribute independently of recruiting Mps1. While not depicted here, the RZZ complex promotes expansion of the fibrous corona, the outermost region of the kinetochore, into crescents and rings that do not contain Bub1; thus, Bub1 and RZZ may recruit spatially distinct pools of Mad1-Mad2 complexes.