Nuclear phosphatidylinositol-specific hhospholipase c is involved in the g2/m phase transition

Protein kinase C (PKC) is activated at the nucieus during the G₂ phase of cell cycle,where it is required for mitosis.However, the trechanisms controlling cell cycle dependent activaion of nuclear PKC are not known. Many isoforrns of CKI are known to exist in ;> number of organisms, and splice variants add additional diversity to the family. The functional différences between the various isofornis and splice variant are just beginning to b<> understood. For example, ("KI< is not able to phosphorylat.e various protein substrates as effectively as CKIo. This lack of catalytic activity is the result of autophosphorylation events resulting in inhibition of the CKI< en/ynie. Inhibition is relieved in vitro by either dophosphorylation of CKIc with any of a number of cellular phosphatases or removal of the portion of the en/yme carboxy-terrninal to the kinase domain by cloning or partial proteolysis. The project goal was to identify the autophosphorylated residues responsible for inhibition of CKIe. Techniques included phospho-amino acid analysis. HPLC and peptide sequencing to identify phosphorylated peptides. Putative phosphory lated residues on such peptides will be changed using site-directed mutagenesis and the resulting changes in the activity of the enzyme observed. Although the majority of CKIf autophosphorylation occurs on the carboxy terminal tail of the kinase, autophosphorylation sites in the amino-terminus, in the kinase domain, appear to mediate inhibiton via an intramolecular mechanism.