Protein kinase A phosphorylates cyclin D1 at three distinct sites within the cyclin box and at the C-terminus.

Cyclin D1 is a nuclear phosphoprotein that is thought to play a major role in the control of G0/G1-->S progression. The fact that its expression is not tightly regulated during cell cycle progression suggests that its activity might be modulated by post-translational mechanisms. In the present study, we show that out of five serine-threonine kinases tested only protein kinase A (PKA) was able to phosphorylate cyclin D1 in vitro. In agreement with this observation, forskolin treatment, but not TPA stimulation, led to increased phosphorylation of cyclin D1 in vivo. Phosphoamino acid analysis and two-dimensional phosphopeptide mapping of wild-type and truncated cyclin D1 proteins showed that PKA phosphorylates three distinct serine residues in cyclin D1 at positions 90, 197 and 234. Serine-90 is located within the cyclin box, raising the possibility that phosphorylation of cyclin D1 might play a role in regulating the interaction with other proteins.