Engineered Inhibitor Sensitivity in the WPD Loop of a Protein Tyrosine Phosphatase

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Small molecules that can be used to turn off the activities of specific cellular proteins are essential tools for chemical biology. Few such compounds are known, however, and they are particularly difficult to identify for members of large protein families. Here, we present a method for insertion of a chemical "off switch" into a catalytically essential loop region (the "WPD loop") of a protein tyrosine phosphatase (PTP). Using a combination of point mutations and amino acid insertions, we have engineered variants of T-cell PTP (TCPTP) that possess cysteine-rich WPD loops. The engineered WPD loops, which contain sequences that appear in no wild-type PTP, confer upon TCPTP the ability to bind a cell-permeable small molecule (the biarsenical fluorescein derivative, FlAsH) that is not an inhibitor of wild-type PTPs. We have identified sites in TCPTP's WPD loop that can be modified to display FlAsH-binding cysteine residues without disrupting TCPTP's inherent PTP activity, as assayed with either small-molecule or phosphorylated-peptide PTP substrates. Upon addition of the FlAsH ligand, however, the activities of the mutants drop dramatically. Inhibition of the FlAsH-sensitized TCPTP mutants is rapid and specific; and strong FlAsH sensitivity was observed in mutants that contain as few as two cysteine point mutations in their engineered WPD loops. Our results show that relatively conservative substitutions can be used to engineer precise small-molecule control of PTP activity. Moreover, since all known classical PTPs utilize the WPD-loop mechanism targeted in this study, it is likely that a substantial fraction of the PTP superfamily can be sensitized through an analogous approach.