A novel murine Staufen isoform modulates the RNA content of Staufen complexes

Mouse Staufen (mStau) is a double-stranded RNA-binding protein associated w ith polysomes and the rough endoplasmic reticulum (RER). We describe a nove l endogenous isoform of mStau (termed mStau(i)) which has an insertion of s ix amino acids within dsRBD3, the major double-stranded RNA (dsRNA)-binding domain. with a structural change of the RNA-binding domain, this conserved and widely distributed isoform showed strongly impaired dsRNA-binding abil ity. In transfected cells, mStau(i) exhibited the same tubulovesicular dist ribution (RER) as mStau when weakly expressed; however, when overexpressed, mStau(i) was found in large cytoplasmic granules. Markers of the RER coloc alized with mStau(i)-containing granules, showing that overexpressed mStau( i) could stiff be associated with the RER. Cotransfection of mStau(i) with mStau relocalized overexpressed mStau(i) to the reticular RER, suggesting t hat they can form a complex on the RER and that a balance between these iso forms is important to achieve proper localization. Coimmunoprecipitation de monstrated that the two mStau isoforms are components of the same complex i n vivo. Analysis of the immunoprecipitates showed that mStau is a component of an RNA-protein complex and that the association with mStau(i) drastical ly reduces the RNA content of the complex. We propose that this new isoform , by forming a multiple-isoform complex, regulates the amount of RNA in mSt au complexes in mammalian cells.