N-(Cyclohexanecarboxyl)-O-Phospho-[formula omitted]-Serine, a Minimal Substrate for the Dual-Specificity Protein Phosphatase IphP

Three dual-specific phosphatases [DSPs], IphP, VHR, and Cdc14, and three protein-tyrosine phosphatases [PTPs], PTP-1B, PTP-H1, and Tc-PTPa, were challenged with a set of low molecular weight phosphoesters to probe the factors underlying the distinct substrate specificities displayed by these two mechanistically homologous families of protein phosphatases. It was observed that β-naphthyl phosphate represented an excellent general substrate for both PTPs and DSPs. While DSPs tended to hydrolyze α-naphthyl phosphate at rates comparable to that of the β-isomer, the PTPs PTP-1B and Tc-PTPa did not. PTP-H1, however, displayed high α-naphthyl phosphatase activity. Intriguingly, PTP-H1 also displayed much higher protein-serine phosphatase activity in vitro, 0.2–0.3% that toward equivalent tyrosine phosphorylated proteins, than did PTP-1B or Tc-PTPa. The latter two PTPs discriminated between the serine- and tyrosine-phosphorylated forms of two test proteins by factors of ≥104–106. While free phosphoserine represented an extremely poor substrate for all of the DSPs examined, the addition of a hydrophobic “handle” to form N-(cyclohexanecarboxyl)-O-phospho-l-serine produced a compound that was hydrolyzed by IphP with high efficiency, i.e., at a rate comparable to that of free phosphotyrosine or p-nitrophenyl phosphate. VHR also hydrolyzed N-(cyclohexanecarboxyl)-O-phospho-l-serine (1 mM) at a rate approximately one-tenth that of β-naphthyl phosphate. None of the PTPs tested exhibited significant activity against this compound. However, N-(cyclohexanecarboxyl)-O-phospho-l-serine did not prove to be a universal substrate for DSPs as Cdc14 displayed little propensity to hydrolyze it.