The state of phosphorylation of Wee1 and Cdc25 is the result of the balance between the protein kinase and the protein phosphatase activities that regulate them. By inhibiting the protein phosphatases, okadaic acid causes Wee1 and Cdc25 to accumulate in their phosphorylated forms as shown in the figure below. Since this change activates MPF, Wee1 and Cdc25 must have originally been present in the extract in their nonphosphorylated forms. Thus active Wee1 kinase is nonphosphorylated as is inactive Cdc25 phosphatase. C. Okadaic acid has no effect on Cdc2 phosphorylation because it is phosphorylated on a tyrosine residue.
Tyrosine phosphatases are unaffected by okadaic acid.
The decrease in Cdc2 phosphorylation is a consequence of the change in activation of Wee1 kinase and Cdc25 phosphatase. As soon as some active MPF appears it would begin to phosphorylate Wee1 and Cdc25, inactivating the kinase and activating the phosphatase. This in turn would further decrease the activity of Wee1 kinase and further increase the activity of Cdc25 phosphatase, leading to still more MPF activity. Thus the initial appearance of a little MPF activity would rapidly lead to its complete activation. This sort of activation is referred to as a positive feedback loop.