The nicotinamide mononucleotide adenylyl transferase (NMNAT/Nma1) modulates phosphate-sensing (PHO) signaling independent of its NAD+ synthesis activity in Saccharomyces cerevisiae.

Abstract

Regulation of NAD+ metabolism is interconnected with multiple nutrient-sensing pathways and cellular processes. The phosphate (Pi)-sensing (PHO) signaling pathway contributes to NAD+ degradation, and PHO-responsive genes are reciprocally regulated in a NAD+-dependent manner. In this study, we examine whether the NAD+ biosynthetic enzyme Nma1 has a direct role in modulating PHO signaling. We show that Nma1 physically interacts with Pho4, a transcription factor translocating to the nucleus to activate PHO-responsive genes during Pi depletion. Overexpression of NMA1 or its catalytically inactive variant significantly reduces Pi depletion-induced Pho4 nuclear localization and the activation of PHO-responsive genes, indicating the NAD+ synthesis activity of Nma1 is dispensable in the downregulation of PHO signaling. Interestingly, mutating the C-terminal domain of Nma1, which is required for the ATPase chaperone activity of mammalian NMNATs, fails to decrease Pho4 nuclear localization and the expression of PHO-responsive genes. Moreover, loss of Nma1 increases Pho4 nuclear localization under moderate Pi-depleted conditions, suggesting that cells lacking Nma1 are more sensitive to Pi availability alterations. These results support that Nma1 can moderate PHO activation by maintaining Pho4 in the cytoplasm. Our findings uncover a novel regulatory mechanism for PHO signaling, and this regulation may help coordinate NAD+ metabolism with Pi homeostasis.