Our results illustrate how gene duplication and sub-functionalization shape the workings of an essential molecular network. This pseudo-symmetric interaction underpins a template–copy relationship crucial for kinetochore–microtubule attachment and SAC signaling. We demonstrate that kinetochore localization of BubR1 relies on direct hetero-dimerization with Bub1 at a pseudo-symmetric interface. This gain-of-function BubR1 mutant cannot sustain a functional checkpoint. Grafting a short motif of Bub1 onto BubR1 promotes Bub1-independent kinetochore recruitment of BubR1. We demonstrate that Bub1, but not BubR1, enhances binding of Bub3 to phosphorylated kinetochores. Because both Bub1 and BubR1 hetero-dimerize with Bub3, a targeting adaptor for phosphorylated kinetochores, the molecular basis for such sub-functionalization is unclear. Subsequent sub-functionalization established subordination: Bub1, recruited first to kinetochores, promotes successive BubR1 recruitment.
Bub1 and BubR1, SAC components, originated from duplication of an ancestor gene. The spindle assembly checkpoint (SAC) monitors and promotes kinetochore–microtubule attachment during mitosis.
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