The TPxxS phospho-dependent degron in Human G1/S-specific cyclin-E1 (CCNE1) that serves as a recognition motif for the SCF(FBW7) ubiquitin ligase, which mediates the degradation of cyclin E within catalytically active cyclin E-Cdk2 complexes and is not involved in the turnover of unbound or unphosphorylated cyclin E