A directly labeled TR-FRET assay for monitoring phosphoinositide-3-kinase activity

Phosphoinositide 3-kinases (PI3Ks) comprise a family of kinases that transfer the terminal phosphate of adenosine triphosphate to phosphoinositides at the 3-hydroxyl of the inositol ring to form phosphoinositide (3,4,5) triphosphate (PIP3). The PI3Ks have been shown to play key roles in cell growth, motility, morphology, and survival and thus are of interest as targets in anti-inflammatory and anti-oncogenic drug development. To facilitate identification of novel and selective inhibitors of PI3Ks, we have developed a TR-FRET assay that uses directly labeled reagents. The assay makes use of the high affinity binding of phosphoinositides to a Pleckstrin homology (PH) domain in the general receptor for phosphoinositides 1 (Grp1) protein. It monitors PIP3 produced from the enzymatic reaction by measuring its competition with Bodipy-FL-labeled PIP3 for binding to Terbium chelate-labeled Grp1. By using directly labeled reagents, this assay configuration offers higher sensitivity and faster binding/dissociation kinetics than existing non-radioactive assays, which are critical for competitive assay formats. The assay is homogenous, robust (Z' = 0.88), and simple and, thus, compatible with high throughput screening (HTS) processes.