CELL-CYCLE STAGE-SPECIFIC PHOSPHORYLATION OF THE EPSTEIN-BARR-VIRUS IMMORTALIZATION PROTEIN EBNA-LP

EBNA-LP is a viral nuclear oncoprotein implicated in the immortalizati on of B lymphocytes by Epstein-Barr virus. An analysis of EBNA-LP migr ation on polyacrylamide gels was performed with protein derived from t he X50-7 lymphoblastoid cell line blocked by hydroxyurea or aphidicoli n at the G(1)/S phase of the cell cycle or by nocodazole at the G(2)/M phase. More slowly migrating species of EBNA-LP were detected in G(2) /M phase-arrested cell extracts. Release from nocodazole G(2)/M block or treatment with phosphatase caused the more slowly migrating species of EBNA-LP to disappear. Analyses of (PO43-)-P-32-labeled EBNA-LP pro tein immunoprecipitated from the drug-synchronized cells showed that p hosphorylated EBNA-LP was present throughout the cell cycle but that p hosphorylation increased in G(2) and was maximal at G(2). Phosphoamino acid analysis revealed that all phosphorylation was on serine residue s only. The ability of EBNA-LP to be phosphorylated by p34(cdc2) kinas e and casein kinase II exclusively on serines implicates these enzymes as being potentially involved in EBNA-LP phosphorylation.