The Arabidopsis BUB1/MAD3 family protein BMF3 requires BUB3.3 to recruit CDC20 to kinetochores in spindle assembly checkpoint signaling

ABSTRACT Mitosis is monitored by the spindle assembly checkpoint (SAC) which remains active until all chromosomes have their kinetochores attached to microtubules originated from opposite spindle poles. Plants produce both highly conserved and sequence-diverged SAC components, so it has been largely unknown how SAC activation leads to the assembly of these proteins at the unattached kinetochores to prevent anaphase onset in plants. In Arabidopsis thaliana , the noncanonical BUB3.3 protein was detected at kinetochores throughout mitosis, unlike MAD1 and the plant specific BUB1/MAD3 family protein BMF3 that associated with unattached chromosomes only. However, BUB3.3 was required to arrest mitotic progression when one or more chromosomes did not congress at the metaphase plate. Nevertheless, BUB3.3 was not required for the kinetochore localization of other SAC components and vice versa . BUB3.3 specifically interacted with BMF3 in a novel region containing two internal repeats that were not required for the kinetochore localization of BMF3. This interaction was important for BMF3 to recruit the CDC20 protein to the unattached kinetochores. Our results showed that the Arabidopsis BUB3.3 protein functioned in the activation of BMF3 for CDC20 recruitment, rather than the recruitment of BMF proteins as what has been found in fungi and animals, in order to arrest mitosis at prometaphase. Therefore, activated SAC resulted in BUB3.3-independent localization of BMF3 and MAD1 to kinetochores and BUB3.3-dependent licensing of BMF3 for CDC20 recruitment in A . thaliana . SIGNIFICANCE STATEMENT Mitotic progression into anaphase is monitored by spindle assembly checkpoint that is poorly understood in plants. Using Arabidopsis thaliana as a reference system, we discovered a novel interaction pattern centered at the BUB1 and MAD3 protein BMF3 at kinetochores. A noncanonical isoform of the evolutionarily conserved BUB3 family protein BUB3.3 interacted with two novel internal repeats in BMF3 for recruiting the CDC20 protein to unattached kinetochores in order to inhibit its function in anaphase onset. Hence, our work sheds light on how spindle assembly checkpoint operates in flowering plants that produce the highly conserved BUB3.3 protein but highly divergent BMF proteins.