Sanchez-Casalongue ME, Lee J, Diamond A, Shuldiner S, Moir RD, Willis IM
J Biol Chem. American Society for Biochemistry and Molecular Biology; 2015 Jan 28;:jbc.M114.626523
Transcriptional regulation of ribosome and tRNA synthesis plays a central role in determining protein synthetic capacity and is tightly controlled in response to nutrient availability and cellular stress. In Saccharomyces cerevisiae, the regulation of ribosome and tRNA synthesis was recently shown to involve the Cdc-like kinase Kns1 and the GSK-3 kinase Mck1. In this study, we explored additional roles for these conserved kinases in processes connected to the Target of Rapamycin Complex 1 (TORC1). We conducted a synthetic chemical-genetic screen in a kns1∆ mck1∆ strain and identified many novel rapamycin-hypersensitivity genes. Gene ontology analysis showed enrichment for TORC1-regulated processes (vesicle-mediated transport, autophagy and regulation of cell size) and identified new connections to protein complexes including the protein kinase CK2. CK2is considered to be a constitutively active kinase and in budding yeast, the holoenzyme comprises two regulatory subunits, Ckb1 and Ckb2, and two catalytic subunits, Cka1 and Cka2. We show that Ckb1 is differentially phosphorylated in vivo and that Kns1 mediates this phosphorylation when nutrients are limiting and under all tested stress conditions. We determined that the phosphorylation of Ckb1 does not detectably affect the stability of the CK2 holoenzyme but correlates with the reduced occupancy of Ckb1 on tRNA genes after rapamycin treatment. Thus, the differentialoccupancy of tRNA genes by CK2 is likely to modulate its activation of RNA polymerase III transcription. Our data suggest that TORC1, via its effector kinase Kns1, may regulate the association of CK2 with some of its substrates by phosphorylating Ckb1.