Highlights d In early embryos, the Drosophila G3BP Rasputin (RIN) is polysome associated d RIN binds over 550 mRNAs, which are short, stable, and highly translated d In rin mutants, steady-state levels of its target mRNAs are reduced d Tethering RIN or human G3BPs enhances the expression of a luciferase reporter mRNA

32G3BP RNA-binding proteins are important components of stress granules (SGs). Here we 33 analyze the role of Drosophila G3BP, Rasputin (RIN), in unstressed cells, where RIN is not SG 34 associated. Immunoprecipitation followed by microarray analysis identified over 550 mRNAs 35 that copurify with RIN. The mRNAs found in SGs are long and translationally silent. In contrast, 36we find that RIN-bound mRNAs, which encode core components of the transcription, splicing 37 and translation machinery, are short, stable and highly translated. We show that RIN is 38 associated with polysomes and provide evidence for a direct role for RIN and its human 39 homologs in stabilizing and upregulating the translation of their target mRNAs. We propose that 40 when cells are stressed the resulting incorporation of RIN/G3BPs into SGs sequesters them away 41 from their short target mRNAs. This would downregulate the expression of these transcripts, 42 even though they are not incorporated into stress granules. 43 44 KEY WORDS 45RNA-binding protein, post-transcriptional regulation, mRNA translation, mRNA stability, stress 46