The PP2A^B56 phosphatase promotes the association of Cdc20 with APC/C in mitosis

PP2A comprising B56 regulatory subunit isoforms (PP2A^B56) is a serine/threonine phosphatase essential for mitosis. At the kinetochore, PP2A^B56 both stabilizes microtubule binding and promotes silencing of the spindle assembly checkpoint (SAC) through its association with the SAC protein BubR1. Cells depleted of the B56 regulatory subunits of PP2A are delayed in activation of Cdc20-containing APC/C (APC/C^Cdc20), which is an essential step for mitotic exit. It has been hypothesized that this delay arises from increased production of the mitotic checkpoint complex (MCC), an APC/C^Cdc20 inhibitor formed at unattached kinetochores through SAC signaling. In contrast to this prediction, we show that depletion of B56 subunits does not increase the amount or stability of the MCC. Rather, delays in APC/C^Cdc20 activation in B56-depleted cells correlate with impaired Cdc20 binding to APC/C. Stimulation of APC/C^Cdc20 assembly does not require binding between PP2A^B56 and BubR1, and thus this contribution of PP2A^B56 towards mitotic exit is distinct from its functions at kinetochores. PP2A^B56 associates with APC/C constitutively in a BubR1-independent manner. A mitotic phosphorylation site on Cdc20, known to be a substrate of PP2A^B56, modulates APC/C^Cdc20 assembly. These results elucidate the contributions of PP2A^B56 towards completion of mitosis.