We have found that modification of rat PC12 cells with pertussis toxin resulted in an ∼50% inhibition of a protein phosphatase 2A‐like phosphatase. Protein phosphatase 2A (PP2A) is a major cellular serine/threonine‐specific protein phosphatase. Treatment of extracts from pertussis toxin‐modified PC12 cells with either immobilized alkaline phosphatase or Ca 2+ reversed this inhibition. Reactivation of the PP2A‐like phosphatase in Ca 2+ appears to result from the dephosphorylation of a protein by the Ca 2+ /calmodulin‐dependent protein phosphatase calcineurin. The PP2A‐like phosphatase in extracts from pertussis toxin‐modified PC12 cells eluted from a Mono Q column at a higher ionic strength than did the PP2A‐like phosphatase in extracts from control cells. After incubation in Ca 2+ , the PP2A‐like phosphatase in extracts from pertussis toxin‐modified cells eluted from a Mono Q column at the same ionic strength as did the PP2A‐like phosphatase in extracts from control cells. These results indicate that the effect of pertussis toxin on this PP2A‐like activity results from the phosphorylation of either one of the subunits of the PP2A‐like phosphatase or a protein that when phosphorylated binds to and inhibits this phosphatase. Pertussis toxin modification did not result in the phosphorylation of the catalytic subunit of PP2A. Because phosphorylation regulates the activities of many enzymes and cell surface receptors, a pertussis toxin‐induced decrease in PP2A activity could alter signaling pathways and other cellular processes in which G proteins are not directly involved.
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Chen et al. (1998) studied this question.