Nuclear actin and actin-binding proteins in the regulation of transcription and gene expression

Abstract

Nuclear actin is involved in the transcription of all three RNA polymerases, in chromatin remodeling and in the formation of heterogeneous nuclear ribonucleoprotein complexes, as well as in recruitment of the histone modifier to the active gene. In addition, actin-binding proteins (ABPs) control actin nucleation, bundling, filament capping, fragmentation and monomer availability in the cytoplasm. In recent years, more and more attention has focused on the role of actin and ABPs in the modulation of the subcellular localization of transcriptional regulators. This review focuses on recent developments in the study of transcription and transcriptional regulation by nuclear actin, and the regulation of muscle-specific gene expression, nuclear receptor and transcription complexes by ABPs. Among the ABPs, striated muscle activator of Rho signaling and actin-binding LIM protein regulate actin dynamics and serum response factor-dependent muscle-specific gene expression. Functionally and structurally unrelated cytoplasmic ABPs interact cooperatively with nuclear receptor and regulate its transactivation. Furthermore, ABPs also participate in the formation of transcription complexes.

Fig. 1

Model for actin-hnRNP U-mediated control of pol II transcription elongation. Actin may modulate several steps in pol II transcription initiation and elongation, either as a monomer or as a polymer. Actin may modulate transcription as a monomeric component of transcription pre-initiation, chromatin remodeling and hnRNP complexes. During transcription elongation, actin may be recruited to the elongating transcription machinery via the hyperphosphorylated CTD and then to the nascent RNP, where actin in complex with the hnRNP U can facilitate recruitment of PCAF or P2D10 to the active gene. Formation of actin filaments in the proximity of the Pol II CTD may help establish a network of interactions between the various factors necessary for transcription elongation and pre-mRNA processing.

Fig. 2

A model for the involvement of STARS and ABLIM in actin dynamics and SRF-dependent transcription.

Fig. 3

Regulation of androgen receptor gene transcription by actin-binding proteins.

Fig. 4

The model of Fil-I participated in transcription regulation. Fli-I protein can bind to components of the p160 coactivator complex (p160 and CARM1), which contains histone acetylating (CBP/p300) and methylating (CARM1) activities. Fli-I can also bind to actin and the actin-like protein BAF53, both of which are components of the ATP-dependent nucleosome-remodeling complex SWI/SNF.