N-Myc Downregulated Gene 1 Is a Phosphorylated Protein in Mast Cells

We previously demonstrated that the in vitro maturation of mouse immature bone marrow-derived mast cells into a mature connective tissue mast cell-like phenotype is accompanied by a marked induction of N-myc downregulated gene (NDRG) 1, a cytosolic protein with unknown function. Here we show that NDRG1 undergoes phosphorylation in mast cells. Recombinant NDRG1 was phosphorylated by calmodulin kinase-II, protein kinase (PK) A and PKC in vitro. Deletion of the C-terminal tandem repeats of NDRG1 resulted in increased phosphorylation by PKA and PKC, but not by calmodulin kinase-II. Furthermore, NDRG1 was phosphorylated on serine and threonine residues in mast cells, a process that was accelerated transiently following cell activation. Pharmacologic studies using kinase-specific inhibitors demonstrated that this NDRG1 phosphorylation in mast cells depended on calmodulin kinase-II and PKA, but not PKC. Collectively, our results indicate that NDRG1 is a multiphosphorylated protein in mast cells, and that the kinetics of increased NDRG1 phosphorylation parallels signaling events leading to exocytosis.