|Budget Amount *help
¥3,500,000 (Direct Cost : ¥3,500,000)
Fiscal Year 1998 : ¥1,500,000 (Direct Cost : ¥1,500,000)
Fiscal Year 1997 : ¥2,000,000 (Direct Cost : ¥2,000,000)
Cyclin E-Cdk2 is an evolutionary conserved G1 Cyclin-dependent kinase (CDK) that is specifically required for the initiation of DNA replication. The activity of cyclin E-Cdk2 in G1 phase is mainly regulated by CDK inhibitors (CKLs), like p21 and p27. An E6-AP like HECT (homologous to the E6-AP carboxyl terminus) family gene encoding a putative E3 ubiquitin-protein ligase was isolated by a two-hybrid screening using both cyclin E and p21, as the bait. This gene named Ceb 1 (cyclin E-binding protein) possesses RCC1 (regulator of chromosome condensation) -like repeats besides the HECT domain, and encodes a 117-kd cytoplasmic protein that can interact with various cyclins in mammalian cells, and can be phosphorylated by the Cdk2 kinases. Expression of Cebi was specifically detected in testis and ovary among various human tissues and highly elevated Ceb1 expression was exclusively limitted in tissue culture cells containing defective tumor suppressor proteins, p53 and pRB.Consistently, inactivation of these tumor suppressors by viral oncoproteins like SV4O T antigen and human papillomavirus(HPV) type 16, E6 or E7 markedly induced the Ceb1 message in primary human fibroblasts. Furthermore, the expression of Ceb1 was induced by the treatment with histone deacetylase inhibitors and through cell's population doubling, and the Cebl expression induced by SV4O T antigen was repressed by ectopic overexpression of the adenovirus p53.
Cyclin E expression levels is inversely correlated with the Cebl expression levels. Cells expressing Ceb1 at high levels exhibited low cyclin E protein levels, and vice versa. Forced overexpression of the cyclin E message in cells expressing Ceb1 at high levels did not allow overexpression of cycilin E protein unlike cells expressing Ceb1 at low levels.Thus, Ceb1 likely mediates the regulation of cyclin E expression in testis and in p53 and pRB defective cells.