Keywords:

Amino Acid Sequence, Binding Sites, Cell Cycle Proteins, Cell Line, Cyclin E, Cyclin-Dependent Kinase Inhibitor Proteins, Dimerization, F-Box Proteins, Humans, Models, Biological, Molecular Sequence Data, Phosphopeptides, Phosphorylation, Protein Binding, Protein Structure, Tertiary, S-Phase Kinase-Associated Proteins, Saccharomyces cerevisiae, Saccharomyces cerevisiae Proteins, Sequence Homology, Amino Acid, Serine, SKP Cullin F-Box Protein Ligases, Structure-Activity Relationship, Substrate Specificity, Threonine, Transfection, Ubiquitin, Ubiquitin-Protein Ligases

Abstract:

<p>The ubiquitin-mediated proteolysis of cyclin E plays a central role in cell-cycle progression, and cyclin E accumulation is a common event in cancer. Cyclin E degradation is triggered by multisite phosphorylation, which induces binding to the SCF(Fbw7) ubiquitin ligase complex. Structures of the Skp1-Fbw7 complex bound to cyclin E peptides identify a doubly phosphorylated pThr380/pSer384 cyclin E motif as an optimal, high-affinity degron and a singly phosphorylated pThr62 motif as a low-affinity one. Biochemical data indicate that the closely related yeast SCF(Cdc4) complex recognizes the multisite phosphorylated Sic1 substrate similarly and identify three doubly phosphorylated Sic1 degrons, each capable of high-affinity interactions with two Cdc4 phosphate binding sites. A model that explains the role of multiple cyclin E/Sic1 degrons is provided by the findings that Fbw7 and Cdc4 dimerize, that Fbw7 dimerization enhances the turnover of a weakly associated cyclin E in vivo, and that Cdc4 dimerization increases the rate and processivity of Sic1 ubiquitination in vitro.</p>