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RNA-binding proteins contribute to the formation of ribonucleoprotein (RNP) granules by phase transition, but regulatory mechanisms are not fully understood. Conserved fission yeast NDR (Nuclear Dbf2-Related) kinase Orb6 governs cell morphogenesis in part by spatially controlling Cdc42 GTPase. Here we describe a novel, independent function for Orb6 kinase in negatively regulating the recruitment of RNA-binding protein Sts5 into RNPs to promote polarized cell growth. We find that Orb6 kinase inhibits Sts5 recruitment into granules, its association with processing (P) bodies, and degradation of Sts5-bound mRNAs by promoting Sts5 interaction with 14-3-3 protein Rad24. Many Sts5-bound mRNAs encode essential factors for polarized cell growth, and Orb6 kinase spatially and temporally controls the extent of Sts5 granule formation. Disruption of this control system affects cell morphology and alters the pattern of polarized cell growth, revealing a role for Orb6 kinase in the spatial control of translational repression that enables normal cell morphogenesis. DOI: http://dx.doi.org/10.7554/eLife.14216.001, eLife digest Living cells can grow to adopt a range of different shapes. For example, fission yeast cells maintain their rod-like shape by growing from their ends and then splitting in the middle to produce two new cells of an equal size. Like many other cells, fission yeast often responds to shortages of nutrients or other environmental stressors by growing more slowly or stopping its growth altogether. One way that this stress response is achieved is by preventing certain growth-promoting proteins from being made by storing or degrading the RNA molecules that are needed to make these proteins. Fission yeast uses an enzyme called Orb6 to control its growth and its overall shape. This enzyme is a kinase, meaning that it adds phosphate groups on to other proteins. Orb6 controls cell growth in part by defining the scope of action of an important growth control factor, Cdc42. This is done by preventing the localization of a molecule that activates Cdc42 at the wrong places of the cell membrane, such as, for example, the cell sides. Now, Nuñez et al. show that the Orb6 enzyme also controls growth via a completely separate mechanism. Orb6 prevents an RNA-binding protein called Sts5 from being recruited into clusters of RNA molecules and proteins called granules, and directs Sts5 to interact with another protein called Rad24 instead. Since the RNA molecules in the granules tend to end up being degraded, preventing Sts5 from being recruited to the granules protects the RNA molecules that bind to Sts5. Many Sts5-bound RNAs encode proteins required for cell growth, and in this manner Orb6 promotes the production of these Sts5-controlled proteins to encourage normal cell growth. In the future, Nuñez et al. would like to determine how Orb6 recognizes and responds to environmental signals to control cell growth. Further studies could also explore how Sts5 and Orb6 kinase affect the assembly of RNA-protein particles that have been implicated in diseases in humans. DOI: http://dx.doi.org/10.7554/eLife.14216.002