The protein phosphatase Siw14 controls caffeine-induced nuclear localization and phosphorylation of Gln3 via the type 2A protein phosphatases Pph21 and Pph22 in Saccharomyces cerevisiae.

The Saccharomyces cerevisiae Siw14, a tyrosine phosphatase involved in the response to caffeine, participates in regulation of the phosphorylation and intracellular localization of Gln3, a GATA transcriptional activator of nitrogen catabolite repression-sensitive genes. In Δsiw14 cells, the phosphorylation level of Gln3 is decreased and the nuclear localization of Gln3 is stimulated by caffeine. However, the mechanism by which Siw14 controls the localization and function of Gln3 remains unclear, although the nuclear localization of Gln3 is known to be induced by activation of the type 2A phosphatases (PP2As) Pph21 and Pph22, and the type 2A-related phosphatase Sit4. In this study, we show that the increased nuclear localization of Gln3 in response to caffeine caused by disruption of the SIW14 gene is dependent on the Sit4 and PP2A phosphatases. We also show that decreased phosphorylation of Gln3 caused by disruption of the SIW14 gene is completely suppressed by deletion of both PPH21 and PPH22, but only partially suppressed by deletion of SIT4. Taking these results together, we conclude that Siw14 functions upstream of Pph21 and Pph22 as an inhibitor of the phosphorylation and localization of Gln3, and that Sit4 acts independently of Siw14.