The human double-stranded (ds) RNA-binding protein Staufen (hStau) is considered to have a role in RNA transport and its localization. By using sedimentation analysis on sucrose gradients, we showed that the Staufen isoform with an apparent molecular mass of 55kDa (Stau55) co-fractionated with ribosomes and associated with both the 40 and 60S ribosomal subunits, suggesting that the Staufen isoform hStau55 plays some role in translation. To map the determinant(s) involved in this association, we generated a series of deletion mutants and analysed their subcellular distribution by cell fractionation and fluorescent immunomicroscopy. Our results demonstrated that multiple determinants promote hStau55—ribosome association via both an RNA-binding-dependent mechanism and protein—protein interaction. The RNA-binding activity of the ds RNA-binding protein domain 3 (dsRBD3) but not that of dsRBD4 is the first determinant. Although necessary for stable association with ribosomes, dsRBD3 alone is not sufficient and needs other determinants as co-factors. Consistently, when expressed together, dsRBD4 and the tubulin-binding domain constitute the minimal Stau55/ribosome protein—protein association domain. This region of Stau55 is sufficient to associate with ribosomes independently, but requires the RNA-binding activity of dsRBD3 for complete association. Thus the results are consistent with a putative role for Stau55 in the regulation of translation.

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