CHARACTERIZATION OF A MUTANT CALCINEURIN A-ALPHA GENE EXPRESSED BY EL4 LYMPHOMA-CELLS

The calmodulin-stimulated phosphatase calcineurin plays a critical rol e in calcium-dependent T-lymphocyte activation pathways. Here, we repo rt the identification of a missense mutation in the calcineurin A alph a gene expressed by EL4 T-lymphoma cells. This mutation changes an evo lutionarily conserved aspartic acid to asparagine within the autoinhib itory domain of the calcineurin A alpha protein. A comparison of wild- type and mutant autoinhibitory peptides indicates that this amino acid substitution greatly reduces inhibition of calcineurin phosphatase ac tivity. Additional peptide inhibition studies support a pseudosubstrat e model of autoinhibitory function, in which the conserved aspartic ac id residue may serve as a molecular mimic of either phosphoserine or p hosphothreonine. Expression of the mutant calcineurin appears to affec t cellular signal transduction pathways, as EL4 cells can be activated by suboptimal concentrations of calcium ionophore in the presence of phorbol esters. Moreover, this phenotype can be transferred to Jurkat T cells by transfection of the mutated calcineurin gene. These finding s implicate a conserved aspartic acid in the mechanism of calcineurin autoinhibition and suggest that mutation of this residue is associated with aberrant calcium-dependent signaling in vivo.