Retracted : Three BUB 1 and BUBR 1/ MAD 3‐related spindle assembly checkpoint proteins are required for accurate mitosis in Arabidopsis

The spindle assembly checkpoint ( SAC ) is a refined surveillance mechanism which ensures that chromosomes undergoing mitosis do not segregate until they are properly attached to the spindle microtubules ( MT ). The SAC has been extensively studied in metazoans and yeast, but little is known about its role in plants. We identified proteins interacting with a MT ‐associated protein MAP 65‐3, which plays a critical role in organising mitotic MT arrays, and carried out a functional analysis of previously and newly identified SAC components. We show that Arabidopsis SAC proteins BUB 3.1, MAD 2, BUBR 1/ MAD 3s and BRK 1 interact with each other and with MAP 65‐3. We found that two BUBR 1/ MAD 3s interacted specifically at centromeres. When stably expressed in Arabidopsis, BRK 1 localised to the kinetochores during all stages of the mitotic cell cycle. Early in mitosis, BUB 3.1 and BUBR 1/ MAD 3.1 localise to the mitotic spindle, where MAP 65‐3 organises spindle MT s. A double‐knockout mad3.1 mad3.2 mutant presented spindle MT abnormalities, chromosome misalignments on the metaphase plate and the production of lagging chromosomes and micronuclei during mitosis. We conclude that BRK 1 and BUBR 1/ MAD 3‐related proteins play a key role in ensuring faithful chromosome segregation during mitosis and that their interaction with MAP 65‐3 may be important for the regulation of MT ‐chromosome attachment.