Co-transcriptional nuclear actin dynamics

Abstract

Actin is a key player for nuclear structure and function regulating both chromosome organization and gene activity. In the cell nucleus actin interacts with many different proteins. Among these proteins several studies have identified classical nuclear factors involved in chromatin structure and function, transcription and RNA processing as well as proteins that are normally involved in controlling the actin cytoskeleton. These discoveries have raised the possibility that nuclear actin performs its multi task activities through tight interactions with different sets of proteins. This high degree of promiscuity in the spectrum of protein-to-protein interactions correlates well with the conformational plasticity of actin and the ability to undergo regulated changes in its polymerization states. Several of the factors involved in controlling head-to-tail actin polymerization have been shown to be in the nucleus where they seem to regulate gene activity. By focusing on the multiple tasks performed by actin and actin-binding proteins, possible models of how actin dynamics controls the different phases of the RNA polymerase II transcription cycle are being identified.

Keywords: Nuclear actin; RNA polymerase; actin polymerization; nuclear structure and function; transcription.

Figure 1. The possible contribution of actin in RNA polymerase II transcription activation. Top panel, monomeric actin interacts with the PSF/NonO complex to recruit the positive elongation factor P-TEFb with the subunit cdk9. This in turn leads to Ser2 phosphorylation within the heptapeptide repeats of the RNA polymerase II CTD. This mechanism promotes RNA polymerase II CTD escape from pausing. Bottom panel, the CTD associated-actin interacts with hnRNP U and this mechanism commits the hyperphosphorylated RNA polymerase II to transcription elongation through recruitment of the HAT PCAF. RNAPII, RNA polymerase II; U, hnRNP U; T, ATP-actin; P-S2, phosphorylated Ser2; P-S5, phosphorylated Ser5.

Figure 2. Hypothetical models depicting actin-based mechanisms during transcription elongation, with respect to a eukaryotic gene. (A) After commitment to elongation, as part of the PSF/NonO complex, actin interacts with hnRNP U to recruit the HAT PCAF. Concomitantly, actin polymers are nucleated through F-actin nucleating factors such as N-WASP and ARP2/3 that are also part of the same multiprotein complex together with PSF/NonO. This mechanism leads to H3K9 acetylation at the exit of the gene promoter, it generates an open chromatin configuration and favors passage of the elongating polymerase through the nucleosome barrier to start transcribing the gene. (B) During transcription elongation the polymerase-associated actin undergoes head-to-tail polymerization. This mechanism is controlled by the F-actin severing activity of cofilin that accompanies the elongating polymerase to maintain a pool of polymerization competent actin. Co-transcriptional actin polymerization occurs throughout the entire length of the transcribed gene and contributes to provide directionality to the elongating polymerase through maintenance of permissive chromatin. RNAPII, RNA polymerase II; U, hnRNP U; T, ATP-actin; P-S2, phosphorylated Ser2; P-S5, phosphorylated Ser5; Ac, acetylated histone; D, ADP-actin; T, ATP-actin; C, cofilin; P, profilin.

Figure 3. A schematic diagram summarizing the RNA polymerase II transcription cycle. Actin is known to be directly involved in PIC formation, escape from pausing and transcription elongation.

Figure 4. The actin nucleocytoplasmic transport cycle. Actin is actively imported into the nucleus in complex with cofilin. Ran-dependent nuclear import requires the import factor importin 9 which interacts with actin-associated cofilin. In the nucleus actin is excluded from heterochromatin and it is mostly found to be coupled to euchromatin and consistently, actin import supports transcription. From the cell nucleus actin is exported in complex with profilin. Active nuclear export of the profilin-actin complex is regulated by exportin 6 which specifically targets profilin. By controlling the nucleocytoplasmic shuttling of actin, cofilin and profilin tightly regulate transcription and gene activity.

Figure 5. Speculative model on the dynamic actin polymerization during transcription. In the initial phases of transcription actin is in a monomeric form. This actin fraction contributes to PIC assembly and to facilitate transcription initiation. Upon commitment of the polymerase enzyme to elongation, actin polymerization accompanies the elongation process in a treadmilling regime which is controlled by cofilin and profilin. This mechanism is reiterated throughout the entire length of the transcribed gene. For termination we speculate that a yet unidentified mechanism at the anchorage point of polymeric actin leads to disassembly of actin polymers and contributes to transcription termination. D, ADP-actin; T, ATP-actin; C, cofilin; P, profilin.