Fig. 1. Catalytic activity of Pin1 and mutant proteins. The PPIase assay was performed in triplicate as described in "Materials and Methods" in the presence of increasing amounts of purified recombinant proteins by monitoring the release of 4-nitroaniline after isomerization of (A) phosphorylated or (B) unphosphorylated substrate. , Pin1; , R68,69L; , C113A.