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Molecular and Cellular Biology, January 2003, p. 92-103, Vol. 23, No. 1
0270-7306/03/$08.00+0     DOI: 10.1128/MCB.23.1.92-103.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.

Sam68 Enhances the Cytoplasmic Utilization of Intron-Containing RNA and Is Functionally Regulated by the Nuclear Kinase Sik/BRK

John H. Coyle,1 Brian W. Guzik,1 Yeou-Cherng Bor,1 Li Jin,1 Lucia Eisner-Smerage,1 Stephen J. Taylor,2 David Rekosh,1 and Marie-Louise Hammarskjöld1*

Myles H. Thaler Center for AIDS and Human Retrovirus Research and Department of Microbiology, University of Virginia, Charlottesville, Virginia 22908,1 Department of Molecular and Cell Biology, University of California at Berkeley, Berkeley, California 947202

Received 6 June 2002/ Returned for modification 28 August 2002/ Accepted 20 September 2002

Cells normally restrict the nuclear export and expression of intron-containing mRNA. In many cell lines, this restriction can be overcome by inclusion of cis-acting elements, such as the Mason-Pfizer monkey virus constitutive transport element (CTE), in the RNA. In contrast, we observed that CTE-mediated expression from human immunodeficiency virus Gag-Pol reporters was very inefficient in 293 and 293T cells. However, addition of Sam68 led to a dramatic increase in the amount of Gag-Pol proteins produced in these cells. Enhancement of CTE function was not seen when a Sam68 point mutant (G178E) that is defective for RNA binding was used. Additionally, the effect of Sam68 was inhibited in a dose-dependent manner by coexpression of an activated form of the nuclear kinase Sik/BRK that hyperphosphorylated Sam68. RNA analysis showed that cytoplasmic Gag-Pol-CTE RNA levels were only slightly enhanced by the addition of Sam68, compared to a 60- to 70-fold increase in the levels of Gag-Pol protein expression. Thus, in this system, Sam68 functioned to enhance the cytoplasmic utilization of RNA containing the CTE. These results suggest that Sam68 may interact with specific RNAs in the nucleus to provide a "mark" that affects their cytoplasmic fate. They also provide further evidence of links between signal transduction and RNA utilization.

* Corresponding author. Mailing address: Myles H. Thaler Center for AIDS and Human Retrovirus Research and Department of Microbiology, University of Virginia, Charlottesville, VA 22908. Phone: (434) 982-1598. Fax: (434) 982-1590. E-mail: mh7g{at}virginia.edu.

Molecular and Cellular Biology, January 2003, p. 92-103, Vol. 23, No. 1
0022-538X/03/$08.00+0     DOI: 10.1128/MCB.23.1.92-103.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.



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