Making ribosomal proteins and associated proteins needed to assemble functional ribosomes is estimated to use about half of the transcriptional capacity in a rapidly growing yeast cell. Regulation of this energy-intensive process in response to environmental signals is mediated through TORC1 (target of rapamycin protein kinase complex 1). Kinase activity of TORC1 regulates activity of the transcriptional activator Sfp1, which functions in coupling cell growth to cell division. To better understand this regulatory pathway, Lempiäinen et al. examined proteins that associated with an Sfp-1 fusion protein. Some of the proteins were components of the TORC1 complex itself. Various experimental approaches—including immunoprecipitation of tagged proteins, in vitro kinase assays, and analysis of phosphorylation site mutants of Sfp1—indicated that Sfp1 was a direct target of phosphorylation by TORC1 and that such phosphorylation was important for nuclear localization and transcriptional activity of Sfp1. Alone, the phosphorylation mutants of Sfp1 did not alter cell growth or cell size, but in combination with deletion of SCH9 (a gene encoding a protein kinase that is the putative ortholog of mammalian ribosomal protein S6 kinase 1 and itself a target of TORC1), cell size and growth rate were reduced. In cells lacking Sfp1, phosphorylation of Sch9 was increased. Thus, the authors propose that feedback regulation from Sfp1 through TORC1 controls Sch9, helping to properly balance the output of this pathway. Another Sfp1-interacting protein that was identified was Mrs6, a protein that functions in intracellular vesicular trafficking. Studies with Spf1 mutants with disrupted interaction with Mrs6 indicated that such interaction was needed for proper nuclear localization of Sfp1 and control of phosphorylation of Sch9 through TORC1. The results emphasize the importance of membrane trafficking in TORC1 signaling. The authors also note intriguing similarities of the functional properties of Sfp1 (for which an ortholog in multicellular organisms is not known) and those of the proto-oncogene c-Myc, which acts to regulate cell size and ribosome biogenesis in higher organisms. Thus, Sfp1’s functions may have implications outside the yeast model system.
H. Lempiäinen, A. Uotila, J. Urban, I. Dohnal, G. Ammerer, R. Loewith, D. Shore, Sfp1 interaction with TORC1 and Mrs6 reveals feedback regulation on TOR signaling. Mol. Cell 33, 704–716 (2009). [Online Journal]