Protein kinase C binding to isolated nuclei and its activation by a Ca2+/phospholipid-independent mechanism

The direct interaction of protein kinase C with the nucleus was examined utilizing endogenous protein phosphorylation and [3H]PDBu binding to detect the enzyme. Rat brain nuclei were relatively rich in phorbol ester receptors whereas liver nuclei contained less than 10% of their brain counterpart. Purified protein kinase C from rat brain could bind to purified rat liver nuclei at 4 degrees C or at 24 degrees C reaching apparent equilibrium by 20 min. The binding was linearly dependent on protein kinase C concentration and required free Ca2+ with an EC50 of 0.5 microM. Chelation of Ca2+ with EGTA resulted in rapid loss of phorbol ester receptors from nuclei. Differential extraction experiments with Triton X-100 and NaCl suggested that about 50% of the acquired phorbol ester receptors were bound to chromatin and 25% were associated with the nuclear matrix. Protein Kinase C bound to nuclei was also able to phosphorylate several endogenous nuclear substrates in a Ca2+/phospholipid-independent reaction. These data suggest that protein kinase C can associate with nuclear components leading to the phosphorylation of nuclear substrates.