The F-box proteins are divided into three classes: Fbxws containing WD40 repeat domains, Fbxls containing leucine-rich repeats, and Fbxos containing either different protein-protein interaction modules or no recognizable motifs [REF].
Phosphorylation of Ser64 and, to a lesser extent, Ser72 of Skp2 contributes to the stabilization of Skp2 by preventing its association with APC/CCdh1; however, Skp2 phosphorylation on these residues is dispensable for its subcellular localization and for Skp2 assembly into an active SCF ubiquitin ligase [REF].