page_title: HMGN
section_title: H1 competition and chromatin remodeling
hierarchical_section_title: HMGN / H1 competition and chromatin remodeling
caption_attribution_description: English: Nucleosome organization schema.
context_page_description: HMGN proteins are members of the broader class of high mobility group chromosomal proteins that are involved in regulation of transcription, replication, recombination, and DNA repair.
  HMGN1 and HMGN2 were discovered by E.W. Johns research group in the early 1970s. HMGN3, HMGN4, and HMGN5 were discovered later and are less abundant. HMGNs are nucleosome binding proteins that help in transcription, replication, recombination, and DNA repair. They can also alter the chromatin epigenetic landscape, helping to stabilize cell identity. There is still relatively little known about their structure and function. HMGN proteins are found in all vertebrates, and play a role in chromatin structure and histone modification. HMGNs come in long chains of amino acids, containing around 100 for HMGN1-4, and roughly 200 in HMGN5. Recent research on the HMGN family is focused on their effect on cell identity, and how reduction of HMGNs relates to induced reprogramming of mouse embryonic fibroblasts.
context_section_description: Nucleosomes serve as the protein core (made from 8 histones) for DNA to wrap around, functioning as a foundation for the larger and more condensed chromatin structures of chromosomes. HMGN proteins compete with Histone H1 (linker histone not part of the core nucleosome) for nucleosome binding sites. Once occupied one protein cannot displace the other. However both proteins are not permanently associated to the nucleosomes and can be removed via post transcriptional modifications. In the case of HMGN proteins, Protein kinase C (PKC) can phosphorylate the serine amino acids in the nucleosome binding domain present in all HMGN variants. This gives HMGNs a mobile character as they are continuously able to bind and unbind to nucleosomes depending on the intracellular environment and signaling.
  Active competition between HMGNs and H1 serve an active role in chromatin remodeling and as result play a role in the cell cycle and cellular differentiation where chromatin compaction and de-compaction determine if certain genes are expressed or not. Histone acetylation is usually associated with open chromatin, and histone methylation is usually associated with closed chromatin.
  With use of ChIP-sequencing it is possible to study DNA paired with proteins to determine what kind of histone modifications are present when the nucleosomes are bound to either H1 or HMGNs. Using this method it was found that H1 presence corresponded to high levels of H3K27me3 and H3K4me3, which means that the H3 histone is heavily methylated suggesting that the chromatin structure is closed. It was also found that HMGN presence corresponded to high levels of H3K27ac and H3K4me1, conversely meaning that the H3 histone methylation is greatly reduced suggesting the chromatin structure is open.
caption: Diagram of nucleosome with bound histone H1
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