High impact information on MAD3

• MAD1 and MAD2 act in a surveillance mechanism that mediates a metaphase delay in response to nonexchange chromosomes, whereas MAD3 acts as a crucial meiotic timer, mediating a prophase delay in every meiosis [1].
• Mad2 and Mad3 coprecipitated with Cdc20 at all stages of the cell cycle [2].
• In vivo, we observed an increased binding of Cdc20p to Mad3p and decreased binding to Hsl1p upon checkpoint activation [3].
• We show that the two regions of homology between Mad3p and Bub1p are crucial for these interactions and identify loss of function mutations within each domain of Mad3p [4].
• Negative-stain electron microscopy shows that Mad3 is an extended molecule ( approximately 200 A long), whereas Bub3 is globular [5].

Biological context of MAD3

• Finally, overexpression of wild-type BUB1 or MAD3 genes leads to chromosome instability [6].
• For the first time, we have systematically studied the dependency of Mad3/BubR1 protein recruitment to kinetochores [7].
• We demonstrate that Mad3p function is required for the overexpression of Mad2p to result in a metaphase arrest [7].
• Mad3/BubR1 phosphorylation during spindle checkpoint activation depends on both Polo and Aurora kinases in budding yeast [8].
• We show that budding yeast Mad3, like its human orthologue BubR1, is a phosphoprotein that is hyperphosphorylated during mitosis and when SAC activation is triggered by microtubule depolymerizing agents, kinetochore defects or lack of kinetochore tension [8].

Associations of MAD3 with chemical compounds

• Accordingly, replacing with alanines five serine residues belonging to Polo kinase-dependent putative phosphorylation sites dramatically reduces Mad3 phosphorylation, suggesting that Mad3 is likely an in vivo target of Cdc5 [8].
• In addition, the NH2-terminal portion (161 amino acids) of hBub1 shows a significant homology to yeast MAD3, a protein also known to be involved in the mitotic checkpoint response pathway [9].

Physical interactions of MAD3

• The checkpoint proteins Mad2 and Mad3/BubR1 bind to Cdc20, although how they inhibit APC(Cdc20) is unclear [3].

Other interactions of MAD3

• The bub1-A78V mutant is of particular interest because it produces a wild-type amount of protein that is mutated in the conserved but uncharacterized Mad3-like region of Bub1p [10].
• In budding yeast, Mad1, Mad2, and Mad3 proteins are equally required for arrest in the presence of damage induced by antimicrotubule drugs or catastrophic loss of spindle structure [11].
• Instead, these motifs are important for Mad3p function in the checkpoint and for binding to Cdc20p [3].

Analytical, diagnostic and therapeutic context of MAD3

• Sequence analysis reveals two regions of Mad3p that are 46 and 47% identical to sequences in the NH(2)-terminal region of the budding yeast Bub1 protein kinase [4].

References