Phosphorylation and activation of cAMP-dependent protein kinase by phosphoinositide-dependent protein kinase

Although phosphorylation of Thr-197 in the activation loop of the catalytic subunit of cAMP-dependent protein kinase (PKA) is an essential step for its proper biological function, the kinase responsible for this reaction in vivo has remained elusive. Using nonphosphorylated recombinant catalytic subunit as a substrate, we have shown that the phosphoinositide-dependent protein kinase, PDK1, expressed in 293 cells, phosphorylates and activates the catalytic subunit of PKA. The phosphorylation of PKA by PDK1 is rapid and is insensitive to PKI, the highly specific heat-stable protein kinase inhibitor. A mutant form of the catalytic subunit where Thr-197 was replaced with Asp was not a substrate for PDK1. In addition, phosphorylation of the catalytic subunit can be monitored immunochemically by using antibodies that recognize Thr-197 phosphorylated enzyme but not unphosphorylated enzyme or the Thr197Asp mutant. PDK1, or one of its homologs, is thus a likely candidate for the in vivo PKA kinase that phosphorylates Thr-197. This finding opens a new dimension in our thinking about this ubiquitous protein kinase and how it is regulated in the cell.