Nuclear DNA helicase II/RNA helicase A binds to filamentous actin

Nuclear DNA helicase 11 (NDH H), also designated RNA helicase A, is a multifunctional protein involved in transcription, RNA processing, and transport. Here we report that NDH H binds to F-actin. NDH H was partially purified from HeLa nuclear extracts by ion-exchange chromatography on Bio-Rex 70 and DEAE-Sepharose. Upon gel-filtration chromatography on Sepharose 4B, partially purified NDH H resolved into two distinct peaks. The first NDH H peak, corresponding to the void volume of Sepharose 4B, displayed coelution with an abundant 42-kDa protein that was subsequently identified as actin. Several nuclear proteins such as RNA polymerase H, the U5 small nuclear ribonucleoprotein (RNP)-associated WD40 protein, and heterogeneous nuclear RNPs (hnRNPs) copurified with NDH II. However, only hnRNPs A1 and C were found together with NDH 11 and actin polymers during gel filtration. NDH II and hnRNP C from the HeLa nuclear extract coeluted with F-actin on Sepharose 4B in an RNase-resistant manner, whereas hnRNP A1 was nearly completely removed from F-actin-associated hnRNP complexes following RNA digestion. The association of NDH II and hnRNP C with F-actin was abolished by gelsolin, an F-actin-depolymerizing protein that fragments actin polymers into oligomers or monomers. Furthermore, NDH 11 co-immunoprecipitated with F-actin and hnRNP C, respectively. In vitro translated NDH 11 coeluted with F-actin on Sepharose 4B, whereas no coelution with F-actin was observed for in vitro translated hnRNP A1 or C1. Binding to F-actin requires an intact C terminus of NDH 11 and most likely a native protein conformation. Electron microscopy indicated a close spatial proximity among NDH 11, hnRNP C, and F-actin within the HeLa nucleus. These results suggest an important function of NDH 11 in mediating the attachment of hnRNP-mRPP RNP complexes to the actin nucleoskeleton for RNA processing, transport, or other actin-related processes.