Phospholipase D (PLD)1 was phosphorylated invivo and by an associated kinase invitro following immunoprecipitation. Both phosphorylation events were greatly reduced in a catalytically inactive point mutant in which the serine residue at position 911 was converted into alanine (S911A). The kinase could be enriched from detergent-extracted brain membranes and bind and phosphorylate PLD1 that was immunoprecipitated from COS-7 cells. Using in-gel kinase assays we determined that the size of the kinase is approximately 40kDa and that PLD1 is more effective than S911A in binding the kinase. Preliminary analysis of the phosphorylation sites on PLD1 suggested that the kinase belongs to the casein kinase 2 (CK2) family. Consistent with this, we found that the kinase could utilize GTP, and could be inhibited by heparin and 5,6-dichloro-1-β-D-ribofuranosylbenzimidazole (DRB). Membrane fractions from Chinese hamster ovary (CHO) cell lines that inducibly express PLD1 contained an endogenous kinase activity that phosphorylated PLD1 using GTP and was inhibited by DRB. Direct evidence that the kinase is CK2 came from observations that immunoprecipitates using PLD1 antibodies contained immunoreactive CK2α, and immunoprecipitates using CK2α antibodies contained immunoreactive PLD1. Co-expression of PLD1 in COS-7 cells with the two recombinant CK2 subunits, α or β, suggests that the association of PLD1 with the kinase is through the β subunit. Supporting this, phosphorylation of PLD1 by purified recombinant CK2α was enhanced by purified recombinant CK2β. Assays measuring PLD1 catalytic activity following phosphorylation by CK2 suggest that this phosphorylation event does not influence PLD1-mediated hydrolysis of phosphatidylcholine invitro.
Both authors contributed equally to this work.