Enzyme helps fold the SAC

Espert et al. identify a phosphatase that helps shut down the spindle assembly checkpoint (SAC) when chromosomes are correctly attached to the spindle.

The SAC prevents cells from entering anaphase until they have verified the connections between spindle microtubules and the chromosomes. The checkpoint forms when the enzyme Mps1 phosphorylates the kinetochore protein Knl1, and this alteration attracts other SAC proteins such as Bub1 and BubR1 to kinetochores. Once all the chromosome–spindle links check out, cells remove the phosphates from Knl1, and the checkpoint shuts down as Bub1 and other components disperse. In yeast, the phosphatase PP1, a member of the phosphoprotein phosphatase (PPP) family, dephosphorylates Knl1, but researchers weren’t sure which enzyme performs the task in mammalian cells.

When Espert et al. dosed mammalian cells with an inhibitor that blocks all members of the PPP family, they found that Bub1 and BubR1 remained on kinetochores even in the absence of Mps1 activity. But when they depleted the different catalytic subunits found in PPP proteins, they found that loss of the PP2A subunit prevented SAC disassembly, whereas loss of the PP1 subunit did not.

The researchers showed that one PPP family member, PP2A-B56, dephosphorylates Knl1 and that cells lacking this enzyme are slow to exit mitosis. PP2A-B56 arrives at the kinetochores with BubR1. Thus, even as the SAC assembles, it’s preparing to disassemble. The results don’t rule out a role for PP1, however. PP2A-B56 might initiate SAC shutdown but allow PP1 to remove the remaining phosphates from kinetochore proteins.