Actin and myosin in transcriptional and post-transcriptional control of gene expression

Abstract: Actin plays a key role in basal gene regulation. In the past decade actin has been found to be a component of chromatin remodeling complexes, ribonucleoprotein particles and to associate with all eukaryotic RNA polymerases. Based on the above discoveries the main objective of this thesis has been to elucidate some of the molecular mechanisms through which nuclear actin controls synthesis and processing of RNA transcripts. In mammals, elongation of pre-mRNA transcripts is regulated by the interaction between actin and the heterogeneous nuclear ribonucleoprotein hnRNP U. In Paper I we investigated the molecular mechanisms underlying their cooperative function. We discovered that actin and hnRNP U interact with the phosphorylated RNA polymerase II carboxy-terminal domain to recruit the histone acetyl transferase PCAF to active genes. This mechanism is required to establish permissive chromatin for efficient transcription elongation. There is emerging evidence that changes in the polymerization state of nuclear actin are important for gene regulation. Along these lines, in Paper II we found that nuclear actin dynamics is necessary for RNA polymerase II mediated transcription. We show that the F-actin severing protein cofilin-1 maintains the pool of monomeric actin to be fed into growing actin polymers and this mechanism is specifically required for elongation of pre-mRNA. Altogether these findings suggest actin polymerization occurs to facilitate migration of elongating RNA polymerase II along active genes. Evidence that the interaction between actin and nuclear myosin 1 (NM1) is important for RNA polymerase I transcription elongation led us to investigate their potential synergy in post-transcriptional control of rRNA biogenesis. In Paper III we found that in nucleoli NM1 associates with rRNA, NM1 becomes incorporated into newly synthesized ribosomal subunits and cooperates with actin for their maturation. We also found that rRNA-associated NM1 interacts with the export receptor CRM1 and the RNA binding nucleoporin Nup153 at the basket of the nuclear pore complex (NPC). We propose that NM1 accompanies newly assembled export-competent ribosomal subunits from nucleolus to NPC, thus modulating both their maturation and export.