A negatively charged amino acid in Skp2 is required for Skp2-Cks1 interaction and ubiquitination of p27Kip1.
Wang W, Ungermannova D, Chen L, Liu X. A negatively charged amino acid in Skp2 is required for Skp2-Cks1 interaction and ubiquitination of p27Kip1. J Biol Chem. 2003 Aug 22; 278(34):32390-6.