Extensive studies characterizing the substrate specificity of Wip1 have shown that it is capable of dephosphorylating phospho-serines or phospho-threonines residing in two different motifs. The first identified motif was the diphosphorylated peptide X−1pTX+1pYX+3, where X−1 can be any amino acid, X+1 is any aliphatic amino acid, and X+3 is any amino acid except proline (46). This motif is found in p38 MAPK and uracil-DNA glycosylase (UNG2), both of which are targeted by Wip1. In addition, the p(S/T)Q motif was identified as a Wip1 target sequence (47). The S/T in this motif is phosphorylated by the PI3K-like kinases such as ATM. By using phosphopeptides and recombinant Wip1 protein, Wip1 was shown to efficiently dephosphorylate p(S/T)Q peptides corresponding to several protein targets of the PI3K-like kinases, including p53 phosphorylated on Serine 15 and Chk1 phosphorylated on Serine 345 (47). Further biochemical analysis showed that the optimal p(S/T)Q phosphopeptide substrate for Wip1 is (D/E)(D/E)X−1p(S/T)QX+2, where X−1 is any amino acid and X+2 is any amino acid except basic amino acids or Proline (47). Indeed, many studies have characterized specific Wip1 targets exhibiting these motifs, which include p38, p53, ATM, gamma-H2AX, Chk1, Chk2, UNG2, Mdm2, MdmX, NF-kappaB, XPA, and XPC (Table 1).
