P-bodies (PBs) are cytoplasmic mRNA-protein (mRNP) granules conserved throughout eukaryotes which are implicated in the repression, storage and degradation of mRNAs. PB assembly is driven by proteins with self-interacting and low-complexity domains. Non-translating mRNA also stimulates PB assembly, however no studies to date have explored whether particular mRNA transcripts are more critical than others in facilitating PB assembly. Previous work revealed that rps28bΔ (small ribosomal subunit-28B) mutants do not form PBs under normal growth conditions. Here, we demonstrate that the RPS28B 3UTR is important for PB assembly, consistent with it harboring a binding site for the PB assembly protein Edc3. However, expression of the RPS28B 3UTR alone is insufficient to drive PB assembly. Intriguingly, chimeric mRNA studies revealed that Rps28 protein, translated in cis from an mRNA bearing the RPS28B 3UTR, physically interacts more strongly with Edc3 than Rps28 protein synthesized in trans. This Edc3-Rps28 interaction in turn facilitates PB assembly. Our work indicates that PB assembly may be nucleated by specific RNA ‘scaffolds’. Furthermore, this is the first description in yeast to our knowledge of a cis-translated protein interacting with another protein in the 3UTR of the mRNA which encoded it, which in turn stimulates assembly of cellular structures.
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