Abstract
Protein serine/threonine phosphatases were implicated in the regulation of circadian rhythmicity in the marine dinoflagellate Gonyaulax polyedra based on the effects of three inhibitors specific for protein phosphatases 1 and 2A (okadaic acid, calyculin A, and cantharidin). Chronic exposure to okadaic acid results in a significant period lengthening, as measured by the bioluminescent glow rhythm, while cantharidin and calyculin A cause large phase delays but no persistent effect on period. Short pulses of the phosphatase inhibitors result in phase delays which are greatest near subjective dawn. Unlike 6-DMAP, a protein kinase inhibitor, okadaic acid, calyculin A, and cantharidin do not block light-induced phase shifts. The inhibitors tested also increase radiolabeled phosphate incorporation into Gonyaulax proteins in vivo and block protein phosphatase 1 and 2A activities in Gonyaulax extracts. This study indicates that protein dephosphorylation catalyzed by protein serine/threonine phosphatases is necessary for proper functioning of the circadian system.