The key step to close the case was to demonstrate directly that phosphorylated Ensa or ARPP19
inhibited PP2A-B55, whereas unphosphorylated forms of these proteins did not. Only the trimeric
holoenzyme proved to be susceptible to the inhibition, and affinity chromatography suggested that
only B55 and not other forms of PP2A bound to phosphorylated Ensa. PP1 and other phosphatases also
failed to show affinity for Ensa.