Phospho-regulation and nucleocytoplasmic trafficking of CrzA in response to calcium and alkaline-pH stress in Aspergillus nidulans.

Tolerance to abiotic stresses by microorganisms require of appropriate signalling and regulatory pathways. Calcineurin phosphatases mediate calcium-dependent signalling pathways which are widely distributed among phylogeny. In Saccharomyces cerevisiae, calcineurin mediates the post-translational modification of downstream effectors, most of them transcription factors, being the best-characterized calcineurin-regulated zinc-finger factor 1, Crz1p. Here we study the signalling process of CrzA, a filamentous fungal Crz orthologue, in response to calcium and ambient-pH alkalinization. In Aspergillus nidulans resting cells CrzA locates in the cytoplasm being excluded from nuclei. CrzA is a phospho-protein and upon calcium, manganese or alkaline-pH stresses, accumulates in nuclei in a calcineurin-dependent manner. Functional analysis of CrzA defined the presence of a nuclear-export and two nuclear-localization signals as well as a PSINVE sequence that constitutes the major calcineurin-docking domain. First 450 amino acids of CrzA contain these functional motifs and in this region is where phosphorylated residues locate. Different phosphorylation steps are identified in CrzA and activities of casein kinase 1 homologue, CkiA, and of glycogen synthase kinase-3β, identified for the first time here as GskA, are involved. The phospho-signalling process and nucleocytoplasmic trafficking of CrzA shows similarities to those described in yeast for Crz1p homologues and of NFATs in mammals.