1. Department of Molecular Biology, MB-7, The Scripps Research Institute, 10550 North Torrey Pines Road, La Jolla, California 92037, USA
2. Department of Oncology/Pathology, CCK R8:03, Karolinska Hospital, 171 76 Stockholm, Sweden

Correspondence to: Steven I. Reed¹ Correspondence and requests for materials should be addressed to S.I.R. (e-mail: Email: sreed@scripps.edu). The GenBank accession number for the hCdc4 cDNA encoding the isoform with M_r 110K is AY049984.

Abstract

Cyclin E, one of the activators of the cyclin-dependent kinase Cdk2, is expressed near the G₁–S phase transition and is thought to be critical for the initiation of DNA replication and other S-phase functions^{1, 2, 3}. Accumulation of cyclin E at the G₁–S boundary is achieved by periodic transcription coupled with regulated proteolysis linked to autophosphorylation of cyclin E⁴. The proper timing and amplitude of cyclin E expression seem to be important, because elevated levels of cyclin E have been associated with a variety of malignancies^{5, 6} and constitutive expression of cyclin E leads to genomic instability⁷. Here we show that turnover of phosphorylated cyclin E depends on an SCF-type protein-ubiquitin ligase that contains the human homologue of yeast Cdc4, which is an F-box protein containing repeated sequences of WD40 (a unit containing about 40 residues with tryptophan (W) and aspartic acid (D) at defined positions). The gene encoding hCdc4 was found to be mutated in a cell line derived from breast cancer that expressed extremely high levels of cyclin E.