Calcineurin, a protein phosphatase found in eukaryotic cells, presents a challenging problem in heterologous protein expression because it is both heterodimeric and posttranslationally modified. In this paper, we describe the cloning of both subunits (catalytic A and regulatory B) of calcineurin from a human cDNA library and their expression at high levels in Escherichia coli. The calcineurin A subunit is expressed as an insoluble glutathione S-transferase fusion protein, while the calcineurin B subunit is soluble upon direct expression. Catalytically active holoenzyme is derived from the separately expressed subunits using a three-step refolding protocol. First, the fusion protein is solubilized, then it is cleaved at the fusion junction with thrombin, and, finally, a catalytically competent calcineurin A:calcineurin B:calmodulin complex is reconstituted by cofolding the separately purified components. In addition, we show that a similar refolding protocol can be applied to a C-terminally truncated form of calcineurin A, which lacks an autoinhibitory and calmodulin-binding domain.
