Actin and nuclear myosin I are associated with RNAP II and function in gene transcription

Abstract

The presence of actin in the nucleus as well as its functions in various nuclear processes has been made clear in the past few years. Actin is known to be a part of chromatin-remodeling complexes BAF, which are required for maximal ATPase activity of the Brg1 component of the BAF complex. Moreover, the essential roles of actin in transcription mediated by RNA polymerases I, II and III have been demonstrated recently. On the other hand, a myosin I isoform, which contains a unique NH2-terminal extension for nucleus localization, has been specifically localized in nucleus. As is well known, myosin I is an actin-binding protein and plays an important role in various cellular activities. Though actin and nuclear myosin I (NM I) have been implicated to play distinct roles in gene expression, there has been no evidence for the actin-myosin interaction that might be involved in gene transcription mediated by RNA polymerase II (RNAP II). Here we show evidence that both actin and NM I are associated with RNAP II in nucleus by using co-localization and co-IP assays, and they may act together on gene transcription. The antibodies against β-actin or NM I can block RNA synthesis in a eukaryotic in vitro transcription system with template DNA comprising the promoter and the coding region of human autocrine motility factor receptor (hAMFR) gene; the antibodies pre-adsorbed with purified actin and NM I have no effect in transcriptional inhibition, indicating that the inhibition of transcription by anti-actin and anti-NM I is specific. These results suggest a direct involvement of actin-myosin complexes in regulating tran- scription. It also implicates that actin and NM I may co-exist in a same complex with RNAP II and the interaction of RNAP II with actin and NM I functions in the RNAP II-mediated transcription.actin, nuclear myosin I, RNAP II, gene transcription RNAP II directed transcription is a complex and highly regulated process in which the chromatin remodeling, the formation of preinitiation complexes, the binding of transcription factors to regulatory regions of DNA and the recruitment of the RNAP II complex to the preinitiation complexes are the prerequisites for transcription initiation, and a huge number of proteins contribute to the process by orchestrated interplays. The presence of actin in nucleus as well as its functions in various nuclear processes such as nucleocytoplasmic transport, chromatin remodeling, transcription, and splicing has been well established in the past few years. Actin is known to be a part of chromatin remodeling complex BAF and required for the maximal ATPase activity of BRG1, a component of the BAF complex [1] . In G. tentans, actin binds to hrp65, a nuclear protein associated with mRNP complexes, and the binding is required for transcription on polytene chromosome [2] . Moreover, the incorporation of bromouridine-triphosphate into nascent transcription could be inhibited by both RNAi-mediated depletion of hnRNP U and disruption of the actin-hnRNP U complex [3] . Importantly, actin could be...