The Mechanism Study Of Actin Involved In The Regulation Of Gene Transcription Elongation

Actin is not only a major cytoskeletal component in all eukaryotic cells but also a nuclear protein that plays a role in gene transcription. Recent reports have shown that in gene transcription, nuclear actin plays a key role as a component of chromatin remodeling complexes, transcription pre-initiation complex and messenger RNP (mRNP) particles associated with all three eukaryotic RNA polymerases (pol). Recent studies have suggested that actin, in direct contact with the transcription apparatus, is required in an early step of transcription that is common to all three eukaryotic RNA polymerases. In addition, there is evidence from in vivo studies that actin is involved in the transcription elongation of classⅡgenes. In this case, actin is bound to a specific subset of premessenger RNA binding proteins, and the actin–messenger RNP complex may constitute a molecular platform for recruitment of histone-modifying enzymes.Actin has now been implicated in transcription by RNA polymerases, but the structural form it adopts in these processes remains unclear. Recently, a claim was made that monomeric nuclear actin plays a role in signal transduction; while a study of RNA polymeraseⅠtranscription has implicated polymeric actin, consorting with an isoform of its classical partner myosin. Moreover, actin in complex with hnRNP U, PCAF, and polⅡcan be coprecipitated by DNaseⅠaffinity chromatography, which, notoriously, has a high affinity for monomeric actin. It now seems reasonable to start thinking about functions for both monomeric and polymeric actin in the nucleus.In this study, actin was found to interact with Cdk9, a catalytic subunit of P-TEFb, in elongation complexes (ECs). Using immunofluorescence and immunoprecipitation assays, Cdk9 was found to bind to G-actin through the conserved Thr-186 in the T-loop. Overexpression and in vitro kinase assays showed that G-actin promotes P-TEFb-dependent phosphorylation of the PolⅡC-terminal domain (CTD). An in vitro transcription experiment revealed that the interaction between G-actin and Cdk9 stimulated PolⅡtranscription elongation. ChIP and immobilized template assays indicated that actin recruited Cdk9 to a transcriptional template in vivo and in vitro. Using cytokine interleukin-6 (IL-6)-inducible p21 gene expression system, we revealed that actin recruited Cdk9 to endogenous gene. Moreover, overexpression of actin and Cdk9 increased histone H3 acetylation and acetylized histone H3 binding to a transcriptional template through the interaction with histone acetyltransferase, p300. Taken together, our results suggested that actin participates in transcription elongation by recruiting Cdk9 for phosphorylation of the PolⅡCTD, and the actin-Cdk9 interaction promotes chromatin remodeling.