Mammalian Staufen Is a Double-Stranded-RNA- and Tubulin-Binding Protein Which Localizes to the Rough Endoplasmic Reticulum

Abstract

Staufen (Stau) is a double-stranded RNA (dsRNA)-binding protein involved in mRNA transport and localization in Drosophila.To understand the molecular mechanisms of mRNA transport in mammals, we cloned human (hStau) and mouse (mStau)staufen cDNAs. In humans, four transcripts arise by differential splicing of the Stau gene and code for two proteins with different N-terminal extremities. In vitro, hStau and mStau bind dsRNA via each of two full-length dsRNA-binding domains and tubulin via a region similar to the microtubule-binding domain of MAP-1B, suggesting that Stau cross-links cytoskeletal and RNA components. Immunofluorescent double labeling of transfected mammalian cells revealed that Stau is localized to the rough endoplasmic reticulum (RER), implicating this RNA-binding protein in mRNA targeting to the RER, perhaps via a multistep process involving microtubules. These results are the first demonstration of the association of an RNA-binding protein in addition to ribosomal proteins, with the RER, implicating this class of proteins in the transport of RNA to its site of translation.

ACKNOWLEDGMENTS

We thank Luis Rokeach (Department of Biochemistry, University of Montreal) for the anticalreticulin antibodies, John Bergeron (McGill University) for the anticalnexin antibodies, and André Royal (Department of Pathology and Cell Biology) for the mouse cDNA library. We thank Luis Rokeach and Lea Brakier-Gingras for comments and discussion and Michael Kiebler for sharing unpublished information.

The first two authors contributed equally to this work.

This work was supported by a Natural Sciences and Engineering Research Council of Canada grant to L.D., a National Health Research Development Program grant to L.D., and a Medical Research Council of Canada grant to I.R.N.