Lap2a is required for MRTF-A-SRF transcriptional activity [ChIP-seq]

Myocardin-related transcription factor A (MRTF-A) in complex with the serum response factor (SRF) regulates the expression of cytoskeletal genes in response to cytoplasmic and nuclear actin dynamics. Different components of nucleoskeleton, besides nuclear actin, also regulate the activity of MRTF-A. Here we extend these studies by showing that lamina-associated polypeptide 2a (Lap2a), the nucleoplasmic isoform of Lap2, is a novel regulator, and a direct binding partner of MRTF-A. Lap2a is a nucleoplasmic protein that interacts with A-type lamins and the retinoblastoma protein (pRb) and contributes to chromatin organization. Unlike other known MRTF-A regulators, Lap2a is not required for MRTF-A nucleo-cytoplasmic shuttling; it functions within the nucleus where it binds MRTF-A directly via its unique C-terminal domain prior to MRTF-A forming the complex with chromatin and SRF. Such interaction affects MRTF-A transcriptional activity since genome-wide analysis revealed reduced binding of MRTF-A to SRF target genes in Lap2a knockout cells compared to control cells that consequently led to impaired expression of target genes. Our studies therefore add another regulatory layer to the control MRTF-A-SRF-mediated gene expression, and broaden the role of Lap2a in transcriptional regulation. Overall design: To study the role of Lap2a in the regulation of MRTF-A activity, we examined MRTF-A-chromatin interaction in Lap2a WT and KO MDFs. We performed chromatin immunoprecipitation (ChIP) followed by deep sequencing (ChIP-seq) of MRTF-A in MDFs under serum starved (0.3% FBS) condition and 45 min after serum stimulation (15% FBS). In addition, Pol II phosphorylated at serine 5 (Pol II S5P) was included to monitor transcription activation of serum-inducible genes.