PHOSPHORYLATION OF THE CORE PROTEIN OF HEPATITIS-B VIRUS BY A 46-KILODALTON SERINE KINASE

Core protein is the major component of the core particle (nucleocapsid ) of human hepatitis B virus. Core particles and core proteins are inv olved in a number of important functions in the replication cycle of t he virus, including RNA packaging, DNA synthesis, and recognition of v iral envelope proteins. Core protein is a phosphoprotein with most, if not all, of the phosphorylation on C-terminal serine residues. In thi s study, we identified a serine kinase activity from the ribosome-asso ciated protein fraction of cytoplasm that could specifically bind and phosphorylate the C-terminal portion of recombinant core protein. This kinase is referred to as core-associated kinase (CAK). CAK could be i nhibited by the kinase inhibitors heparin and manganese ions but not b y spermidine, DRB, H89, or H7, indicating that CAK is distinct from pr otein kinase A and protein kinase C. CAK could be partially purified b y heparin-Sepharose CL-6B and phosphocellulose P11 columns. By using a far-Western assay, three specific proteins, of 46, 35, and 13 kDa, we re shown to interact with the C-terminal part of the core protein, The se three proteins were present only in the eluted fractions that conta ins the CAK activity. An in-gel kinase assay showed that a 46-kDa kina se in the same fraction could bind and phosphorylate the C-terminal pa rt of the recombinant core protein. These results indicate that this 4 6-kDa kinase is most probably CAK. A similar 46-kDa kinase, which exhi bits the same profile of sensitivity to kinase inhibitors as that of C AK, is present in both purified intracellular core particles and extra cellular 42-nm virions, suggesting that CAK is a candidate for the cor e particle-associated kinase.