Ctk1 Function Is Necessary for Full Translation Initiation Activity in

Translation is a fundamental and highly regulated cellular process. Previously, we reported that the kinase and transcription elongation factor Ctk1 increases fidelity during translation elongation in Saccharomyces cerevisiae. Here, we show that loss of Ctk1 function also affects the initiation step of translation. Translation active extracts from Ctk1-depleted cells show impaired translation activity of cappedmRNA, but not mRNA reporters containing the cricket paralysis virus (CrPV) internal ribosome entry site (IRES). Furthermore, the formation of 80S initiation complexes is decreased, which is probably due to reduced subunit joining. In addition, we determined the changes in the phosphorylation pattern of a ribosome enriched fraction after depletion of Ctk1. Thus, we provide a catalogue of phosphoproteomic changes dependent on Ctk1. Taken together, our data suggest a stimulatory function of Ctk1 in 80S formation during translation initiation. Translation initiation is a very intricate and highly dynamicprocess leading to the formation of an elongation competent 80S ribosomal complex at the start codon of the mRNA (1–4). Translation initiation starts with the association of the methionyl initiator tRNA (Met-tRNAi) to the GTP-bound eukaryotic initia-tion factor 2 (eIF2) to form a ternary complex (TC). In Saccharo-myces cerevisiae, this TC is thought to bind another subset of ini-tiation factors to form the multifactor complex (5), which the