The Tor kinases are the targets of the potent antiproliferative and immunosuppressive drug rapamycin. Rapamycin has recently been approved by the FDA as an immunosuppressive drug, and phase III clinical trials are in progress for its use as a novel chemotherapy agent. In both yeast and mammalian cells, rapamycin action is mediated by its association with the peptidyl prolyl isomerase, FKBP12. The rapamycin-FKBP12 complex then binds to and inhibits the functions of the Tor kinases, which were first identified by genetic studies in yeast and subsequently discovered in human cells. The Tor kinases regulate cell proliferation, translation and transcription as well as cellular responses to nutrient availability, including autophagy, ribosome biogenesis, cell differentiation, and mating. The Tor pathway plays a major role in yeast in regulating ribosomal protein (RP), ribosomal RNA, and tRNA gene expression in response to nutrients. In addition, Tor controls expression of nutrient utilization genes and stress responsive genes. Although much is known about the mechanisms by which Tor regulates the expression of nutrient utilization and stress responsive genes, very little is known about how Tor controls RP gene expression. We have shown that Tor activity favors the recruitment of the Esa1 histone acetylase to RP gene promoters coincident with RP gene activation. More recently, studies from our group and another have suggested a possible crosstalk between the Tor and cAMP-PKA pathways in regulating RP gene expression in response to nutrients. Many of the functions of the Tor kinases are mediated via type 2A protein phosphatases (PP2A). In yeast the PP2A-like phosphatase, Sit4, is regulated by its association with Tap42 and a set of four related proteins known as the Saps. Our proposed studies seek to define the roles of the Sap proteins in Tor action, to determine if there is crosstalk between the Tor and cAMP-PKA pathways to control RP gene expression, and to define the molecular mechanisms by which Tor signaling controls recruitment of Esa1 to RP gene promoters. Our goal is to elucidate the mechanisms of rapamycin action, many of which are conserved from yeast to mammals, and thereby, provide the biochemical basis for further development of rapamycin and its derivatives as novel chemotherapeutic agents.
