Abstract
Ku is an abundant nuclear heterodimeric protein composed of 70 and 86 kDa subunits. As an activator of the catalytic subunit of DNA-PK (DNA-dependent protein kinase), Ku plays an important role in DNA repair and recombination. Ku is also involved in actions independent of DNA-PK, such as transcription regulation and telomere maintenance. Although Ku is localized in the cytoplasm under specific cellular conditions, no functions for Ku outside of the nucleus have as yet been reported. In addition to DNA binding, Ku binds specific RNA sequences with high affinity. However, no specific cellular mRNA targets for Ku have been identified. In a yeast three-hybrid system, Ku70 bound to an RNA bait that contained an IRES (internal ribosomal entry site) element. A single band with migration properties similar to those of Ku70 was immunoprecipitated with anti-Ku antibody, using UV cross-linked complexes formed by HeLa cell nuclear extracts and an IRES-containing RNA probe. IRES activity was reduced in Ku80(-/-) cells. Overexpression of Ku proteins stimulated IRES-dependent translation. The present study suggests that Ku binds IRES elements within RNA molecules, and that Ku plays a role in the modulation of IRES-mediated mRNA translation.
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