| Mutagenesis studies show that the rapid turnover of MEF in S phase is dependent on the specific phosphorylation of threonine 643 and serine 648 at the C terminus of MEF by cdk 2 and on the Skp1 / Cul1 / F box ( SCF ) E 3 ubiquitin ligase complex SCF ( Skp 2 ) , which targets MEF for ubiquitination and proteolysis . ^^^ |