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Vol. 18, Issue 4, 1302-1311, April 2007

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Subcellular Relocalization of a Trans-acting Factor Regulates XIAP IRES-dependent Translation

Stephen M. Lewis^*, Anne Veyrier^,,, Nicoleta Hosszu Ungureanu^*, Sophie Bonnal^,,, Stéphan Vagner^,,, and Martin Holcik^*

*Apoptosis Research Centre, Children's Hospital of Eastern Ontario Research Institute, University of Ottawa, Ottawa, Ontario, K1H 8L1, Canada; INSERM U563, Toulouse, F-31000, France; Institut Claudius Régaud, Toulouse, F-31052, France; and Université Toulouse III Paul Sabatier, Toulouse, F-31000, France

Submitted June 12, 2006; Revised January 19, 2007; Accepted January 31, 2007
Monitoring Editor: Marvin P. Wickens

Translation of the X-linked inhibitor of apoptosis (XIAP) proceeds by internal ribosome entry site (IRES)-mediated initiation, a process that is physiologically important because XIAP expression is essential for cell survival under conditions of compromised cap-dependent translation, such as cellular stress. The regulation of internal initiation requires the interaction of IRES trans-acting factors (ITAFs) with the IRES element. We used RNA-affinity chromatography to identify XIAP ITAFs and isolated the heterogeneous nuclear ribonucleoprotein A1 (hnRNP A1). We find that hnRNP A1 interacts with XIAP IRES RNA both in vitro and in vivo and that hnRNP A1 negatively regulates XIAP IRES activity. Moreover, XIAP IRES-dependent translation is significantly reduced when hnRNP A1 accumulates in the cytoplasm. Osmotic shock, a cellular stress that causes cytoplasmic accumulation of hnRNP A1, also leads to a decrease in XIAP levels that is abrogated by knockdown of hnRNP A1 expression. These results suggest that the subcellular localization of hnRNP A1 is an important determinant of its ability to negatively regulate XIAP IRES activity, suggesting that the subcellular distribution of ITAFs plays a critical role in regulating IRES-dependent translation. Our findings demonstrate that cytoplasmic hnRNP A1 is a negative regulator of XIAP IRES-dependent translation, indicating a novel function for the cytoplasmic form of this protein.

The online version of this article contains supplemental material at MBC Online (http://www.molbiolcell.org).

Address correspondence to: Martin Holcik (martin{at}mgcheo.med.uottawa.ca)

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