Abstract:
Specialization of cells needs maintenance of cell type specific gene expression patterns established during cell fate determination. The Trithorax group (trxG) proteins counteract repressive effect of Polycomb group (PcG) complexes and maintain transcriptional memory of active states of key developmental genes. Although, chromatin structure and modifications appear to play a fundamental role in this process, it is not clear how trxG prevents PcG-silencing at target genes. Recently, our lab employed a whole genome RNAi approach to identify novel trxG-like factors involved in maintaining gene activity. In this study, a hitherto unknown role for Drosophila Mask is described which emerged as one of the candidate trxG genes in RNAi screen. This study provides evidence that trxG mediated gene activation requires Mask. The genome-wide binding profile of Mask correlates with known Trithorax binding sites across Drosophila genome. The potential role of Mask in gene activation is substantiated by its overlap with H3K27ac, a hallmark of gene activation deposited by trxG protein CBP. Notably, Mask positively regulates H3K27ac levels and biochemically interacts with CBP. In addition, Mask predominantly associates with actively transcribed genes and its depletion results in downregulation of trxG targets. Importantly, mask mutant strongly suppresses extra sex comb phenotype in Pc mutants, and enhances trx mutant phenotype. Together, this data provides physiological relevance of Mask linked to trxG and explains in detail a previously unknown role for Mask in maintenance of gene activation by trxG.

Interestingly, Mask is known to interact with a histone kinase, Ballchen (Ball), via a muscle sarcomere structural protein known as Obscurin. Recently, our lab also discovered that Ball substantially colocalizes with H3K27ac at trxG target loci and is required to maintain gene activation in Drosophila. In this study, a previously unknown interaction between Ball and CBP is identified. Analysis of genomewide binding profile of Ball and CBP reveals major overlap and their co-localization at actively transcribed genes. Importantly, Ball biochemically interacts with CBP and depletion of Ball results in drastic reduction in H3K27ac similar to mask knockdown. Ball also shows significant overlap with Mask binding on chromatin. Together, these results demonstrate a previously unknown synergy between Ball, Mask and CBP that reveals a potentially new pathway required to maintain gene activation during development.

Publications:

1. Shaukat, A., Khan, M.H.F., Akhtar, J., Haseeb, M.A., Mazhar, K., Umer, Z. and Tariq, M. (2022) Genome-wide binding of Drosophila Mask reveals its role in maintenance of gene activation by Trithorax group. bioRxiv.

2. Shaukat, A., Khan, M.H.F., Ahmad, H., Umer, Z. and Tariq, M. (2021) Interplay Between BALL and CREB Binding Protein Maintains H3K27 Acetylation on Active Genes in Drosophila. Frontiers in cell and developmental biology, 9.

3. Khan, M.H.F., Akhtar, J., Umer, Z., Shaheen, N., Shaukat, A., Munir, M.S., Mithani, A., Anwar, S. and Tariq, M. (2021) Kinome-wide RNAi screen uncovers role of Ballchen in maintenance of gene activation by trithorax group in Drosophila. Frontiers in cell and developmental biology, 9, 458.