Two rat brain Staufen isoforms differentially bind RNA

In neurones, a limited number of mRNAs is found in dendrites, including tra nscripts encoding the microtubule-associated protein 2 (MAP2). Recently, we identified a cis-acting dendritic targeting element (DTE) in MAP2 mRNAs. H ere we used the yeast tri-hybrid system to identify potential trans-acting RNA-binding factors of the DTE. A cDNA clone was isolated that encodes a me mber of a mammalian protein family that is highly homologous to the Drosoph ila RNA-binding protein Staufen. Mammalian Staufen appears to be expressed in most tissues and brain areas. Two distinct rat brain Staufen isoforms, r Stau+I6 and rStau-I6, are encoded by alternatively spliced mRNAs, Both isof orms contain four double-stranded RNA-binding domains (dsRBD). In the large r rStau+I6 isoform, six additional amino acids are inserted in the second d sRBD, Although both isoforms interacted with the MAP2-DTE and various addit ional RNA fragments in an in vitro north-western assay, rStau-I6 exhibited a stronger signal of bound radioactively labelled RNAs as compared with rSt au+I6. Using an antibody directed against mammalian Staufen, the protein wa s detected in somata and dendrites of neurones of the adult rat hippocampus and cerebral cortex. Ultrastructural studies revealed that in dendrites, r at Staufen accumulates along microtubules. Thus in neurones, rat Staufen ma y serve to link RNAs to the dendritic microtubular cytoskeleton and may the reby regulate their subcellular localization.