Local activation of yeast ASH1 mRNA translation through phosphorylation of Khd1p by the casein kinase Yck1p.

In S. cerevisiae, the ASH1 mRNA is localized at the bud tip of late-anaphase cells, resulting in the exclusive sorting of Ash1p to the daughter cell nucleus. While the mechanism behind the localization of this transcript has been well studied, the regulation of its translation is still poorly understood. We now report that the RNA binding protein Khd1 interacts with the ASH1 mRNA localization element E1 and with the C-terminal domain of eIF4G1 to regulate the translation of this transcript. Khd1p reduces translation initiation on the ASH1 mRNA and diminishes Ash1p leakage into the mother cell nucleus. Furthermore, we show that the casein kinase Yck1p phosphorylates Khd1p at the plasma membrane, disrupting the Khd1p-RNA complex and releasing its translational repression on the ASH1 mRNA. This study reveals how, by linking mRNA sorting and translational activation, Khd1p and Yck1p regulate the spatiotemporal expression of a cell fate determinant.