The Whi2-Psr1-Psr2 complex selectively regulates TORC1 and autophagy under low leucine conditions but not nitrogen depletion

Amino acids and ammonia serve as sources of nitrogen for cell growth and were previously thought to have similar effects on yeast. Consistent with this idea, depletion of either of these two nitrogen sources inhibits the target of rapamycin complex 1 (TORC1), leading to induction of macroautophagy/autophagy and inhibition of cell growth. In this study, we show that Whi2 and the haloacid dehalogenase (HAD)-type phosphatases Psr1 and Psr2 distinguish between these two nitrogen sources in <i>Saccharomyces cerevisiae</i>, as the Whi2-Psr1-Psr2 complex inhibits TORC1 in response to low leucine but not in the absence of nitrogen. In contrast, a parallel pathway controlled by Npr2 and Npr3, components of the Seh1-associated complex inhibiting TORC1 (SEACIT), suppress TORC1 under both low leucine- and nitrogen-depletion conditions. Co-immunoprecipitations with mutants of Whi2, Psr1, Psr2 and fragments of Tor1 support the model that Whi2 recruits Psr1 and Psr2 to TORC1. In accordance, the interaction between Whi2 and Tor1 appears to increase under low leucine but decreases under nitrogen-depletion conditions. Although the targets of Psr1 and Psr2 phosphatases are not known, mutation of their active sites abolishes their inhibitory effects on TORC1. Consistent with the conservation of HAD phosphatases across species, human HAD phosphatases CTDSP1 (CTD small phosphatase 1), CTDSP2, and CTDSPL can functionally replace Psr1 and Psr2 in yeast, restoring TORC1 inhibition and autophagy activation in response to low leucine conditions.