The kinase domain was expressed in E.coli fused with GST, and purified. I conducted kinase reaction with ATP, and tried to use phostag to separate the mock (no ATP added) and treated sample, but no difference was observed.
Can you provide some details on your purification step? The Phos-tag system works great for many phosphorylated proteins but some others is a little more tricky. I think it's something as simple as the tertiary confirmation of some proteins that physically block the binding site of the PT molecule.