RNA helicase A (RHA) is a multifunctional protein involved in various nuclear processes such as transcription and RNA export. It is believed that the interacting factors play important roles in determining the functional specificity of RHA. Here we show that RHA directly interacts with double-stranded (ds) nucleic acids (NAs) and assembles complexes with topoisomerase II�. First, electrophoresis mobility shift assays demonstrate that RHA interacts with dsDNAs of different lengths ranging from 15 to 104 bp. Secondly, the binding of RHA to closed circular dsDNA stimulates the relaxation reaction catalyzed by either calf thymus topoisomerase I or HeLa topoisomerase IIa. Thirdly, immunoprecipitation, coupled with western blot analysis using anti-RHA and anti-topoisomerase II� antibodies, shows that RHA and topoisomerase II� assemble a complex in the presence of as yet unknown RNA molecules and additional protein factors such as Ubc9. Our observation suggests physical and functional interaction between RHA and topoisomerase II�, which, perhaps, play important roles in regulating chromatin structure. The putative role of RHA-topoisomerase II� complex in RNA polymerase II-mediated transcription is discussed.
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