Phosphorylation of Ser72 does not regulate the ubiquitin ligase activity and subcellular localization of Skp2.

Skp2 is the substrate binding subunit of the SCF(Skp2) ubiquitin ligase, which plays a key role in the regulation of cell cycle progression. The activity of Skp2 is regulated by the APC(Cdh1), which targets Skp2 for degradation in early G(1) and prevent premature S phase entry. Overexpression of Skp2 leads to dysregulation of the cell cycle and is commonly observed in human cancers. We have previously shown that Skp2 is phosphorylated on Ser64 and Ser72 in vivo, and that these modifications regulate its stability. Recently, two studies have proposed a role for Ser72 phosphorylation in the cytosolic relocalization of Skp2 and in the assembly and activity of SCF(Skp2) ubiquitin ligase complex. We have revisited this question and analyzed the impact of Ser72 phosphorylation site mutations on the biological activity and subcellular localization of Skp2. We show here that phosphorylation of Ser72 does not control Skp2 binding to Skp1 and Cul1, has no influence on SCF(Skp2) ubiquitin ligase activity, and does not affect the subcellular localization of Skp2 in a panel of cell lines.