HMGN (High-Mobility Group Nucleosome-binding) proteins are members of the broader class of high-mobility group (HMG) chromosomal proteins that are involved in regulation of transcription, replication, recombination, and DNA repair.
HMGN1 and HMGN2 (initially designated HMG-14 and HMG-17 respectively) 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. HMGNs come in long chains of amino acids, containing around 100 for HMGN1-4, and roughly 200 in HMGN5. A study was done with knockout mice to see the effect if any that HMGN proteins play on a full organism level. This resulted in the mice showing increasing sensitivity to UV radiation when having less than normal levels of HMGN(2). This would indicate that HMGN might facilitate repair of UV damage. The same increase in sensitivity was observed in mice when exposed to gamma radiation, however the cellular processes that repair DNA in either case are drastically different, leading to an inconclusive state whether HMGN proteins facilitate DNA repair in vivo.
HMGN1 and HMGN2 do not co-localize within living cells.]]
HMGN proteins are part of broader group of proteins referred to as High Mobility group chromosomal (HMG) proteins. This larger group was named this for their high electrophoretic mobility in polyacrylamide gels and is differentiated into 3 distinct but related groups, one of them being HMGN proteins. HMGN family can be further divided into specific proteins, these being HMGN1, HMGN2, HMGN3, HMGN4, and HMGN5. The overall sizes of the proteins vary to each specific one, but HMGN1-4 average 100 amino acids. NMR evidence shows that reducing compaction occurs when the proteins targets the main elements that are responsible for the compactions of the chromatin.
HMGN 4
The discovery of HMGN4 was done by GenBank during a database search and identified it as a "new HMGN2 like transcript", indicating that HMGN4 is closely related to HMGN2.
H1 competition and chromatin remodeling
thumb|189x189px|Diagram of nucleosome with bound histone H1
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. HMGN1 expression is elevated during initial stages of eye development in progenitor cells, but is decreased in newly formed and fated cells, such as lens fiber cells. HMGN2 in contrast stays elevated in both embryonic and adult eye cells. HMGN3 was found to be especially elevated at 2 weeks (for an adult mouse) in the inner nuclear and ganglion cells. This shows there is an uneven distribution of HMGNs in pre-fated and adult cells.
Brain / CNS development
thumb|220x220px|Oligodendrocyte differentiation is HMGN reliant
In human brain development HMGNs have been shown to be a critical component of neural differentiation and are elevated in neural stem cells (neural progenitor cells). For example, in a knock down study, loss of HMGN1,2&3 resulted in lower population of astrocyte cells and higher population of neural progenitor cells.
In oligodendrocyte differentiation HMGNs are critical, since when HMGN1&2 are both knocked out the population of oligodendrocytes in spinal tissue was reduced 65%. However, due to functional compensation this effect is not observed when only HMGN1 or HMGN2 are knocked. This observation if not just correlation. With ChIP-seq analysis it is shown that chromatin modeling at the OLIG1&2 genes (transcription factors involved in oligodendrocyte differentiation) is in an open conformation and has HMGNs bound to the nucleosomes.
It can be inferred that this redundancy is actually beneficial as the presence of at least one HMGN variant vastly improves tissue differentiation and development. These findings are summarized in the figure to the right.
See also
- High-mobility group