Interaction amongst calcineurin subunits. Stimulatory and inhibitory effects of subunit B on calmodulin stimulation of subunit A phosphatase activity depend on Mn2+ exposure of the holoenzyme prior to its dissociation by urea.

Calcineurin was dissociated into subunits A and B by 6 M urea in the presence (method A) and absence (method B) of MnCl2 and dissociated subunits were isolated by gel filtration in urea in the absence (method B) or presence (method A) of MnCl2. Phosphatase activity was associated with the A subunit isolated by either method. The phosphatase activity (nmol/mg) of subunit A isolated by method A was greater (2-5-fold) than by method B. Mn2+ increased subunit A phosphatase and calmodulin further increased the enzyme activity. Subunit B isolated by method A or B increased Mn2+ + calmodulin stimulated subunit A phosphatase prepared by method B but interestingly and unexpectedly inhibited such stimulated activity of the subunit A prepared by method A. These results imply the tightly bound cation (in our case, most likely Mn2+) with subunit A dramatically and differentially influences the effects of two Ca2+-binding proteins, calmodulin and subunit B, on the subunit A phosphatase.