Sfp1 integrates TORC1 and PKA activity towards yeast ribosome biogenesis
Target of Rapamycin Complex 1 (TORC1) and Protein Kinase A (PKA) are two major regulators of cell growth in Saccharomyces cerevisiae, coupling nutrient availability with resource-intensive anabolic processes such as ribosome biogenesis. Even though TORC1 and PKA signaling converge on several shared targets, little is known on how these targets integrate signals from the two pathways. This is the case for Sfp1, a transcriptional activator of hundreds of ribosomal protein and ribosome biogenesis genes. To disentangle the roles of PKA and TORC1 in Sfp1 regulation, we constructed a large collection of Sfp1 (phospho)mutants and studied their intracellular localization and phosphorylation. Besides the already known TORC1 phosphorylation sites, we discovered that Sfp1 contains two PKA sites and a functional Nuclear Export Signal (NES) which is regulated by TORC1 and PKA. We further showed that TORC1 and PKA regulate Sfp1 phosphorylation independently of each other, with loss of activity of either pathway being sufficient to relocalize the protein from the nucleus to the cytoplasm, and the C-terminal zinc fingers being necessary for the responsiveness of Sfp1 to TORC1 and PKA inputs. This work contributes to our understanding of how cells regulate their growth by monitoring the outputs of multiple nutrient-sensing pathways.