Protein kinase D is a downstream target of protein kinase Cθ¹

Protein kinase D (PKD/PKCμ) immunoprecipitated from either COS-7 cells or Jurkat T lymphocytes transiently transfected with a constitutively active mutant of PKCθ AE (PKCθAE) exhibited a marked increase in basal activity. In contrast, coexpression of constitutively active mutant of PKCζ does not induce PKD activation in both types of cells. PKCθAE does not induce kinase activity in immunocomplexes of PKD kinase-deficient mutants PKDK618N or PKDD733A. PKD activation in response to PKCθAE signaling was completely prevented by treatment with the protein kinase C (PKC) inhibitors, GF I or Ro 31-8220, or by mutation of Ser-744 and Ser-748 to Ala in the kinase activation loop of PKD. Our results show that PKD is a downstream target of the θ isoform of PKC in both COS-7 cells and lymphocytes. The regulation of PKD by PKCθ reveals a new pathway in the signaling network existing between multiple members of the PKC superfamily and PKD.