The onset of M-phase is regulated by a mechanism common to all eukaryotic cells. Entry into M-phase is determined by activation of the p34cdc2 protein kinase which requires p34cdc2 dephosphorylation and association with cyclin.

Universal control mechanism regulating onset of M-phase

Nuclear targeting sequences--a consensus?

Two interdependent basic domains in nucleoplasmin nuclear targeting sequence: Identification of a class of bipartite nuclear targeting sequence

In vitro disassembly of the nuclear lamina and M phase-specific phosphorylation of lamins by cdc2 kinase

HEPATITIS B virus (HBV) DNA frequently integrates into the genome of human primary liver cancer cells1–4, but the significance of this integration in liver carcinogenesis is still unclear. Here we report the cloning of a single HBV integration site in a human hepatocellular carcinoma at an early stage of development, and of its germline counterpart. The normal locus was found to be transcribed into two polyadenylated messenger RNA species of 1.8 and 2.7 kilobases. We have isolated a complementary DNA clone from a normal adult human liver cDNA library which has an open reading frame with a coding capacity for a protein of 432 amino acids and relative molecular mass 48,536. The strong homology of the C-terminal half of the protein to the A-type cyclins of clam5and Drosophila6 identifies it as a human cyclin A. The cyclin A gene has several exons, and the HBV integration occurs within an intron. As cyclins are important in the control of cell division7–17, the disruption of a cylin A gene by viral insertion might contribute to tumorigenesis.

Hepatitis B virus integration in a cyclin A gene in a hepatocellular carcinoma.

Simian virus 40 large tumour antigen (T) is a replication origin binding protein required for viral DNA synthesis. Unphosphorylated T antigen is deficient in promoting DNA replication in vitro but can be activated by phosphorylation at residue threonine 124 by the cdc2 protein kinase. This observation demonstrates that T is regulated by phosphorylation and provides a model for cdc2 function in the control of DNA replication.

Yasha Gluzman

Papers

Phosphorylation of large tumour antigen by cdc2 stimulates SV40 DNA replication.