Calcineurin Subunits A and B Interact to Regulate Growth and Asexual and Sexual Development in Neurospora crassa

Calcineurin is a calcium/calmodulin dependent protein phosphatase in eukaryotes that consists of a catalytic subunit A and a regulatory subunit B. Previous studies in the filamentous fungus Neurospora crassa had suggested that the catalytic subunit of calcineurin might be an essential protein. We generated N . crassa strains expressing the A ( cna-1 ) and B ( cnb-1 ) subunit genes under the regulation of P tcu-1 , a copper-responsive promoter. In these strains, addition of bathocuproinedisulfonic acid (BCS), a copper chelator, results in induction of cna-1 and cnb-1 , while excess Cu 2+ represses gene expression. Through analysis of these strains under repressing and inducing conditions, we found that the calcineurin is required for normal growth, asexual development and female fertility in N . crassa . Moreover, we isolated and analyzed cnb-1 mutant alleles generated by repeat-induced point mutation (RIP), with the results further supporting roles for calcineurin in growth and fertility in N . crassa . We demonstrated a direct interaction between the CNA-1 and CNB-1 proteins using an assay system developed to study protein-protein interactions in N . crassa .