Tripartite regulation of Gln3p by TOR, Ure2p, and phosphatases

Gln3p is a GATA-type transcription factor responsive to different nitrogen nutrients and starvation in yeast Saccharomyces cerevisiae, Recent evidence has linked TOR signaling to Gln3p. Rapamycin causes dephosphorylation and nuclear translocation of Gln3p, thereby activating nitrogen catabolite repr essible-sensitive genes. However, a detailed mechanistic understanding of t his process is lacking, In this study, we show that Tor1p physically intera cts with Gln3p, An intact TOR kinase domain is essential for the phosphoryl ation of Gln3p, inhibition of Gln3p nude ar entry and repression of Gln3p-d ependent transcription. In contrast, at least two distinct protein phosphat ases, Pph3p and the Tap42p-dependent phosphatases, are involved in the acti vation of Gln3p. The yeast pro-prion protein Ure2p binds to both hyper- and hypo-phosphorylated Gln3p. In contrast to the free Gln3p, the Ure2p-bound Gln3p is significantly resistant to dephosphorylation, Taken together, thes e results reveal. a tripartite regulatory mechanism by which the phosphoryl ation of Gln3p is regulated.