Regulation of transcription by SRF and MRTF

The serum response factor (SRF) is a transcription factor involved in the regulation of cell proliferation and migration. At cytoskeletal genes, SRF acts cooperatively with the actin-regulated myocardin-related transcription factors MRTF-A and MRTF-B. In addition to regulating MRTFs’ subcellular localization, G-actin also controls their nuclear activities. Previous work showed that nuclear accumulation of MRTF-A in the absence of an activating signal is sufficient for its association with genomic loci but not for target gene activation, demonstrating a repressive effect of nuclear actin on MRTF activity. However, the exact mechanism is unclear. The data presented below demonstrates that G-actin interferes with ternary complex formation between SRF and MRTF on DNA. In response to G-actin depletion, MRTF is recruited to target gene promoters and activates gene expression, whereas increasing the concentration of G-actin inhibits MRTF recruitment to target promoters. We used inhibition of the Crm1 nuclear export receptor and reconstitution of MRTF-A/MRTF-B null cells with a constitutively nuclear MRTF- NLS to induce MRTF nuclear accumulation in the absence of an activating signal. Under resting G-actin levels, nuclear MRTF is recruited to target promoters, albeit at reduced levels, and induces non-productive transcription. TTseq and RNAseq experiments demonstrate that while RNA Polymerase II is engaged in elongation, no pre-mRNA accumulates. This inhibited transcriptional state correlates with aberrant Pol II CTD phosphorylation, Mtr4 recruitment and degradation of the nascent transcripts by the nuclear exosome. Inhibition of phosphorylation at Ser7 of the Pol II CTD is sufficient to induce Mtr4-mediated degradation of MRTF- dependent transcripts under induced conditions.