Cell Morphogenesis Proteins Are Translationally Controlled through UTRs by the Ndr/LATS Target Ssd1

Eukaryotic cells control their growth and morphogenesis to maintain integrity and viability. Free-living cells are further challenged by their direct interaction with the environment and in many cases maintain a resilient cell wall to stay alive under widely varying conditions. For these organisms, stringent and highly localized control of the cell wall's remodeling and expansion is crucial for cell growth and reproduction. In the budding yeast Saccharomyces cerevisiae the RNA binding protein Ssd1 helps control cell wall remodeling by repressing translation of proteins involved in wall expansion. Ssd1 is itself negatively regulated by the highly conserved Ndr/LATS protein kinase Cbk1. We sought to identify mRNA regions that confer Ssd1-mediated translational control. After validating a GFP reporter system as a readout of Ssd1 activity we found that 3' untranslated regions of the known Ssd1 targets CTS1, SIM1 and UTH1 are sufficient for Cbk1-regulated translational control. The 5' untranslated region of UTH1 also facilitated Ssd1-mediated translational control in a heterologous context. The CTS1 and SIM1 3' untranslated regions confer Ssd1 binding, and the SIM1 3' untranslated region improves Ssd1 immunoprecipitation of the endogenous SIM1 transcript. However, SIM1's 3' untranslated region is not essential for Ssd1-regulated control of the message's translation. We propose that Ssd1 regulates translation of its target message primarily through UTRs and the SIM1 message through multiple potential points of interaction, permitting fine translational control in various contexts.