Multipartite Fluorogenic Sensors for Monitoring Tyrosine Phosphatase Activity

Fluorogenic substrates are essential tools for studying the activity of many enzymes including the protein tyrosine phosphatases (PTPs). Here, we have taken the first step toward the development of genetically encodable sensors for PTP activity using fluorescent and fluorogen‐activating proteins. The Fluorescence‐Activating and absorption Shifting Tag (FAST) is a small protein that becomes fluorescent upon binding to a small molecule dye. We demonstrate that FAST protein can be used as a sensor for PTP‐mediated dephosphorylation of phosphorylated dye molecules. Phosphorylated 4‐hydroxybenzylidene rhodanine (pHBR) is not able to bind to the FAST protein and induce fluorescence, but provides a sensitive assay for PTP activity, readily detecting 100 pM concentrations of PTP1B in the presence of FAST with a kcat value of 19±1 s−1 and a KM value of 93±3 μM. In addition, while phosphorylation of the C‐terminal peptide of split GFP does not result in appreciable change in fluorescence of the reconstituted protein, phosphorylation of the C‐terminal peptide of the split FAST protein abrogates fluorescence. Upon PTP‐mediated dephosphorylation of the C‐terminal peptide, the ability of the N‐ and C‐terminal components to form a fluorescent complex with the small molecule dye is restored, leading to fluorescence. The Fluorescence‐Activating and absorption‐Shifting Tag (FAST) protein provides a highly sensitive readout of protein tyrosine phosphatase activity. Phosphorylation of either the small molecule fluorogen required for FAST fluorescence or the C‐terminal peptide fragment of splitFAST, abrogates fluorescence of the protein. Phosphatase‐mediated dephosphorylation of these substrates results in significant increases in fluorescence, proportional to phosphatase activity.