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Molecular and Cellular Biology, August 2000, p. 5592-5601, Vol. 20, No. 15
0270-7306/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.

A Novel Murine Staufen Isoform Modulates the RNA Content of Staufen Complexes

Thomas Duchaîne,1 Hui-Jun Wang,2 Ming Luo,1 Sergey V. Steinberg,1 Ivan R. Nabi,2 and Luc DesGroseillers1,*

Departments of Biochemistry1 and Pathology and Cell Biology,2 University of Montreal, Montreal, Quebec, Canada H3C 3J7

Received 21 December 1999/Returned for modification 11 February 2000/Accepted 2 May 2000

Mouse Staufen (mStau) is a double-stranded RNA-binding protein associated with polysomes and the rough endoplasmic reticulum (RER). We describe a novel endogenous isoform of mStau (termed mStaui) which has an insertion of six 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 ability. In transfected cells, mStaui exhibited the same tubulovesicular distribution (RER) as mStau when weakly expressed; however, when overexpressed, mStaui was found in large cytoplasmic granules. Markers of the RER colocalized with mStaui-containing granules, showing that overexpressed mStaui could still be associated with the RER. Cotransfection of mStaui with mStau relocalized overexpressed mStaui to the reticular RER, suggesting that they can form a complex on the RER and that a balance between these isoforms is important to achieve proper localization. Coimmunoprecipitation demonstrated that the two mStau isoforms are components of the same complex in vivo. Analysis of the immunoprecipitates showed that mStau is a component of an RNA-protein complex and that the association with mStaui drastically 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 mStau complexes in mammalian cells.

* Corresponding author. Mailing address: Department of Biochemistry, University of Montreal, P.O. Box 6128, Station Centre Ville, Montreal, QC, Canada H3C 3J7. Phone: (514) 343-5802. Fax: (514) 343-2210. E-mail: desgros{at}bcm.umontreal.ca.

Molecular and Cellular Biology, August 2000, p. 5592-5601, Vol. 20, No. 15
0270-7306/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.


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