Overexpression, purification and characterization of Dictyostelium calcineurin A

The catalytic subunit of Ca2+/calmodulin-dependent protein phosphatase (calcineurin A) was overexpressed about 50-fold in Dictyostelium discoideum cells transformed with a vector containing the cDNA for D. discoideum calcineurin A under control of the actin-6 promoter. In crude lysates from the overexpressing cell line, high Ca2+/calmodulin-stimulated phosphatase activity was detected. Calcineurin A was purified by anion exchange chromatography and calmodulin-Sepharose affinity chromatography, and the enzymatic activity of the isolated protein was characterized. Its phosphatase activity was strictly dependent on the addition of divalent metal ions such as Mg2+ or Mn2+. Disulphide-reducing agents increased the activity more than 10-fold. Ca2+/calmodulin stimulated the activity by a factor of 2.5-5. Despite the high extra Ca2+/calmodulin-dependent phosphatase activity, the overexpressing cell line showed no phenotypic aberrations.