“…Beads were washed 2× with IP-buffer and 1× with KA buffer (40 mM Tris-HCl pH 7.5, 10 mM MgCl 2 , 1 mM DTT), pooled and resuspended in KA buffer. The beads were equally split into eppendorf tubes, 5 mg FSBA-treated cellular protein fraction added and the kinase reaction initiated by adding ‘light’ (SIGMA-Aldrich) or ‘heavy’ 18 O 2 -ATP (Organisch-Chemisches Institut, UZH, Switzerland)29 at a final concentration of 1 mM. After incubation for 30, 90 and 150 min at RT (~23 °C) on a rotary wheel, the reaction was quenched by the addition of solid urea to a final concentration of 8 M. Samples were vortexed and stored on ice.…”