Cyclin E is transcribed in a periodic burst late in G1. Based on both western blot analysis of synchronized cell cultures and immunofluorescence analysis of single cells, cyclin E protein levels peak at the G1/S boundary and decline as cells proceed through S phase (2-4). The decline of cyclin E levels, once transcription is terminated, is linked to the short half-life of cyclin E in S phase cells (12, 13). Cyclin E is degraded via ubiquitin-mediated proteolysis (12, 14). During S phase, cyclin E becomes phosphorylated on T380, which targets it for poly-ubiquitination by the protein-ubiquitin ligase SCFCdc4 and subsequent turnover (12, 14).
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