Studies On The Molecular Mechanism Of Nuclear Actin In Gene Transcription Regulation

Actin is a highly conserved protein, which exists in almost all eukaryotic cells and has been proved to be essential for many cellular functions such as movement, morphology, growth, cytokinesis as well as other crucial events. Over the years actin has been reported to exist in the nucleus, but these results have always been disputed because they could have reflected the contamination due to the high concentration of actin in the cytoplasm. Recent years, based upon ultrstructural, histochemical, and immunofluorescence studies, the presence of actin in the nucleus has been confirmed in different cell types.In contrast to its functions in cytoplasm, the knowledge concerning the role of actin in nucleus is still limited. It has been proposed that actin plays an important role in nuclear export, and actin might be involved in RNA transport and indirectly participate in mRNA processing. It was reported recently that nuclear actin co-localized with protein 4.1, linking actin to nuclear assembly processes. Furthermore, nuclear actin has been found to be associated with chromatin remodeling and histone acetyltransferase complexes, suggesting a role for actin in chromatin remodeling. A number of studies have suggested a role for actin in transcription. In early studies, it had been shown that injection of antibodies against actin or actin-binding proteins in amphibian oocyte nuclei could block the transcription of lampbrush chromosomes. Actin was also reported to co-purify with RNA polymerase II and to be required for efficient in vitro transcription by RNA polymerase II. Recently, it was reported that actin could be recruited to the promoter region of the gene, and is a component of pre-initiation complexes (PICs). However, as a part of PICs, the role of actin in this complex remains to be elucidated.Here, we identified RHA (RNA helicase A) as an actin-interacting protein in PICs. Using immunoprecipitation and immunofluorescence techniques, we showed that RHA associated withβ-actin in the nucleus. GST pull-down assay using different deletion mutants revealed that RGG region of RHA was responsible for the interaction withβ-actin, and this dominant-negative mutant reduced the recruitment of Pol II (RNA polymerase II) into PICs. Moreover, overexpression or depletion of RHA could influence the interaction of Pol II withβ-actin andβ-actin-involved gene transcription regulation. These data suggest that RHA acts as a bridge factor linking nuclearβ-actin with Pol II.