Figure 11.

A model for Gwl function in M phase. MPF phosphorylates many target proteins, including but not limited to Gwl and MPF autoregulatory loop components such as Cdc25 and (kinases [not shown] such as Plx1 and Myt1/Wee1). Phosphorylated, active Greatwall directly, or more likely indirectly, inactivates PP2A/B55δ, thus protecting MPF substrates from dephosphorylation. In this way, Greatwall can simultaneously influence the autoregulatory loop and also function outside of the loop. This scheme implies that M phase (both in mitosis and meiosis) requires feedback mechanisms that not only positively regulate MPF kinase, but also negatively regulate countering “antimitotic” phosphatases including PP2A/B55δ.