Human F-box protein hCdc4 targets cyclin E for proteolysis and is mutated in a breast cancer cell line

Cyclin E, one of the activators of the cyclin-dependent kinase Cdk2, is exp ressed near the G(1)-S phase transition and is thought to be critical for t he initiation of DNA replication and other S-phase functions(1-3). Accumula tion of cyclin E at the G(1)-S boundary is achieved by periodic transcripti on coupled with regulated proteolysis linked to autophosphorylation of cycl in E-4. The proper timing and amplitude of cyclin E expression seem to be i mportant, because elevated levels of cyclin E have been associated with a v ariety of malignancies(5,6) and constitutive expression of cyclin E leads t o genomic instability(7). Here we show that turnover of phosphorylated cycl in E depends on an SCF-type protein-ubiquitin ligase that contains the huma n homologue of yeast Cdc4, which is an F-box protein containing repeated se quences of WD40 (a unit containing about 40 residues with tryptophan (W) an d aspartic acid (D) at defined positions). The gene encoding hCdc4 was foun d to be mutated in a cell line derived from breast cancer that expressed ex tremely high levels of cyclin E.