The role of nuclear envelope proteins BAF-1 and MEL-28 in chromatin organisation

Abstract

BAF-1 (Barrier to Autointegration Factor) is a highly conserved chromatin binding protein implicated in nuclear envelope (NE) breakdown and assembly as well as chromatin compaction. Its NE localisation is interdependent of lamins and LEM-domain proteins (LAP2, emerin, and MAN1). Nevertheless, BAF-1 is also present in the nucleoplasm. Strikingly, a single amino acid substitution in human BAF (Ala12Thr) causes Nestor-Guillermo Progeria Syndrome (NGPS). This illness is manifested at 2 years age and affects a variety of tissues, leading to severe skeletal defects and scoliosis. We have modified the baf-1 locus in Caenorhabditis elegans to mimic the human NGPS mutation (baf-1(G12T)) to elucidate why a mutation in an essential protein expressed throughout development triggers the appearance of symptoms only a few years into childhood after normal embryonic development. We propose that changes in tissue-specific genome organisation are driving forces in NGPS. Our preliminary data show that baf-1(G12T) mutants are hypersensitive to NE perturbations, particularly to modifications affecting lamin/LMN-1 and emerin/EMR-1 proteins. This dramatically affects mitotic chromosome segregation and embryonic viability. Using Bimolecular Fluorescence Complementation, we discovered the nuclear pore protein MEL-28/ELYS as a novel BAF-1 interaction partner. MEL-28 plays a critical role in segregation of meiotic and mitotic chromosomes and NE assembly, whereas we found that MEL-28 associates with active chromatin in post-mitotic cells. Similar to BAF-1, a significant fraction of MEL-28 localises in the nucleoplasm, suggesting that MEL-28 might be directly involved in control of gene expression. In support of this, mel-28 mutants have a delayed larval development and reduced lifespan. We are currently studying the effects of MEL-28 depletion and BAF-1(G12T) mutation on nuclear organisation and gene expression through DamID and RNA-seq.