MEMBRANE TRANSPORT STRUCTURE FUNCTION AND BIOGENESIS
A Human Importin-β Family Protein, Transportin-SR2, Interacts with the Phosphorylated RS Domain of SR Proteins*

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Serine/arginine-rich proteins (SR proteins) are mainly involved in the splicing of precursor mRNA. RS domains are also found in proteins that have influence on other aspects of gene expression. Proteins that contain an RS domain are often located in the speckled domains of the nucleus. Here we show that the RS domain derived from a human papillomavirus E2 transcriptional activator can target a heterologous protein to the nucleus, as it does in many other SR proteins, but insufficient for localization in speckles. By using E2 as a bait in a yeast two-hybrid screen, we identified a human importin-β family protein that is homologous to yeast Mtr10p and almost identical to human transportin-SR. This transportin-SR2 (TRN-SR2) protein can interact with several cellular SR proteins. More importantly, we demonstrated that TRN-SR2 can directly interact with phosphorylated, but not unphosphorylated, RS domains. Finally, an indirect immunofluoresence study revealed that a transiently expressed TRN-SR2 mutant lacking the N-terminal region becomes localized to the nucleus in a speckled pattern that coincides with the distribution of the SR protein SC35. Thus, our results likely reflect a role of TRN-SR2 in the cellular trafficking of phosphorylated SR proteins.

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This work was supported by Academia Sinica and Grant DOH88-HR-812 from the National Health Research Institutes of Taiwan.The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

The first three authors contributed equally to this work.