Regulation of aflatoxin production by Ca(2+)/calmodulin-dependent protein phosphorylation and dephosphorylation

To elucidate Ca(2+)-mediated regulation of aflatoxin production, the status of Ca(2+)/calmodulin-dependent protein phosphorylation and dephosphorylation was investigated employing toxigenic and non-toxigenic strains of Aspergillus parasiticus. Incubation of cytoplasmic extracts with [gamma-(32)P]ATP followed by SDS-PAGE and autoradiography revealed total absence of protein phosphorylation during periods corresponding to aflatoxin production in the toxigenic strain (NRRL 2999). In contrast, protein phosphorylation was unaffected in the non-toxigenic strain (SRRC 255). Aflatoxin production in the toxigenic strain was also accompanied by enhanced (26-fold) activity of calcineurin (calmodulin-dependent protein phosphatase 2B) concomitant with a lowered (6-fold) activity of calmodulin-dependent protein kinase. In addition, the in vitro activity of Ca(2+)/calmodulin-dependent protein kinase was susceptible to dose-dependent inhibition by aflatoxin. Since calcineurin remains active in the absence of phosphorylation by calmodulin-dependent protein kinase, it is suggested that calcineurin-mediated dephosphorylation of regulatory enzymes ensures continued production of aflatoxins.