Immobilized PKA fusion proteins were prepared using purified HaloTag-fusion protein and Magne-HaloTag or HaloLink beads (Promega). In a 1.5mL centrifuge tube, a 200µL slurry of fresh beads was pelleted using a magnetic separator or mini centrifuge and the suspension solution was removed. Beads were then washed with PBS (2 x 500 µL) and resuspended to a total volume of 200µL in PBS. To this bead suspension, a 1 mL solution of purified fusion protein (0.268 mg/mL) was added and incubated on an end-over-end rotator for 4h at room temperature. The loaded beads were then pelleted, supernatant removed by pipette, and subsequently washed with PBS (3 x 1 mL). The loaded beads were resuspended to a final volume of 200µL in a glycerol storage buffer (20 mM Tris, 300 mM NaCl, 1 mM EDTA, 50% glycerol, pH 8.0).