Abstract
To study signal transduction directed towards the cell nucleus and at the nuclear membranes, we investigated the association of protein kinase C (PKC) with nuclear membranes obtained from nuclei isolated from bovine brain. By use of phorbol-ester-binding assays, significant amounts of PKC could be demonstrated in nuclei and nuclear membranes. Nuclear membranes are shown to be able to activate purified PKC. The PKC endogenously present in nuclear membranes appears to be a so-called 'membrane-inserted' form: it is permanently active, still binds phorbol ester, but its activity is no longer dependent on Ca2+ and cannot be activated by phorbol ester. On the other hand, this form of PKC can be inhibited by specific PKC inhibitors. By using histone HIIIS and a specific peptide substrate, it could be shown that after extraction with Triton X-100 the PKC can be stimulated by phospholipid again. Immunoblot analysis with isoenzyme-specific antibodies revealed that the alpha- and gamma-isoenzymes, but not the beta-isoenzyme, are associated with membranes derived from brain nuclei.