Recent work has highlighted the role of RNA-binding proteins in the formation of ribonucleoprotein (RNP) granules by phase transition, but the mechanisms and regulation of this process are not fully understood. Here, we describe a novel function of the conserved NDR kinase Orb6 in inhibiting the localization of mRNA-binding protein Sts5 into RNPs. In fission yeast, Orb6 kinase has an established function in cell morphogenesis through the spatial control of Cdc42 GTPase. In this work, we show that regulation of Sts5 represents a genetically separable mechanism by which Orb6 kinase promotes polarized cell growth.
We find that Orb6 kinase inhibits the formation of Sts5 puncta that often colocalizate with processing (P) bodies by promoting Sts5 interaction with 14-3-3 protein Rad24. In addition, Orb6 kinase prevents degradation of Sts5-bound mRNAs, many of which encode proteins that have been shown to be essential for polarized cell growth. The condensation of Sts5 into cytoplasmic puncta is spatially and temporally controlled by Orb6 kinase, and interfering with this regulatory mechanism disrupts the pattern of polarized cell growth. Our work thus uncovers a role for Orb6 kinase in the spatial control of translational repression during normal cell morphogenesis.