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= HMGN Proteins =

History/Discovery[edit source]

 * The first HMGN proteins (HMGN1 and HMGN2) were identified by E.W. Johns group under the names HMG-14 and HMG-17[1]
 * HMGN1 and HMGN2 are the most abundant and characterized proteins of the HMGN family. In addition to nucleosome binding, they reduce the compactness of chromatin fiber, and enhance transcription of chromatin templates[2][3]

Family[edit source]
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.[4] 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.[5] Whereas the larger HMGN5 proteins are 300+ amino acids long in mice and roughly 200 in length for humans.[6]

HMGN 1 and HMGN 2[edit source]
HMGN1 and HMGN2 are among the most common of the HMGN proteins. The main purpose and function are reducing the compaction of the cellular chromatin by nucleosome binding.[7] NMR evidence shows that reducing compaction occurs when the proteins targets the main elements that are responsible for the compactions of the chromatin.[5] These have an expression rates that correlate to the differentiation of the cells it is present in. Areas that have experienced differentiation have reduced expression levels in comparison to undifferentiated areas, where HMGN1 and HMGN2 are highly expressed.[7]

HMGN3[edit source]
HMGN3 has two variants, HMGN3a and HMGN3b.[5] Unlike the HMGN1 and HMGN2 proteins, both forms of HMGN3 tend to be tissue and development specific.[5]They are only expressed in certain tissues at specific developmental stages. There is no preference to a certain tissue given by the two variants of the HMGN3 proteins. There is equal likelihood that either be present in a certain highly expressed HMGN3 tissue.[7] The brain and the eyes in particular are areas that HMGN3 is heavily expressed as well as in adult pancreatic islet cells.[5] It has been shown that the loss of HMGN3 in mice has led to a mild onset of diabetes due to ineffective insulin secretion.[8]

HMGN 4[edit source]
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.[5] There has been very little research done on HMGN4 proteins. The gene associated with the production of the HMGN4 is located in a region associated with schizophrenia on chromosome 6.[7] Until this point every kind of HMGN has been identified in the vertebrates, but HMGN4 has only been seen and identified in primates.[5] Within humans, HMGN4 has shown high levels of expression in the thyroid, thymus and the lymph nodes.[5]

HMGN 5[edit source]
The most recent addition to the HMGN protein family is of HMGN5. It is larger than the previous HMGNs, containing 300+ amino acids, due to a long C-terminal domain that varies with species, explaining  why mice and humans have a different size of HMGN5.[5] Its biological function is unknown but has shown expression in placental development.[7] There have also been cases where HMGN5 was present in human tumors including, prostate cancer, breast cancer, lung cancer, etc.[5] For this reason, it is thought that HMGN5 might have some link to cancer and might be a potential target for cancer therapy in the future.

HMGN and H1 competition and chromatin remodeling[edit source]
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 H1 linker histone (H1 histone not part of the core nucleosome) for nucleosome binding sites.[9] Once occupied one protein can’t displace the other. However both proteins aren’t permanently associated to the nucleosomes and can be removed via post transcriptional modifications. In the case of HMGN proteins, PKC can phosphorylate the serine amino acids in the nucleosome binding domain present in all HMGN variants.[10] This gives HMGNs a mobile character as they are continuously able to bind and unbind to nucleosomes depending on the intracellular environment and signaling.

Interplay 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-seq 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.[11] 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.[11]

Cell Cycle Dependent binding of HMGN proteins to Chromatin*[edit source]

 * The location of HMGN during mitosis is the subject of several studies[12]
 * Very difficult to date their intranuclear organization during the various stages of of cell cycle remains undetermined[12]
 * Superfamily of abundance and ubiquitous nuclear proteins that bind to chromatin without any known DNA sequence[12]
 * Interactions of all HMGs with chromatin is highly dynamic, proteins move constantly throughout the nucleus[12]
 * Sample nucleosomes for potential binding sites in a “stop and go” manner[12]
 * THe stop step is longer than the go step[12]
 * The superfamily composed of three families, HMGA, HMBG, HMGN[12]
 * HMGA is associated with chromatin throughout the cell cycle, located in the scaffold of the metaphase chromosome[12]
 * Both HMGB and HMGN are associated with the mitotic chromosome[12]
 * Through the use of immunofluorescence studies, live cell imaging, gel mobility shift assays, and bimolecular fluorescence complementation and also by comparing the chromatin binding properties of wild-type and HMGN mutant proteins[12]
 * Conclusion: HMGNs can associate with mitotic chromatin[12]
 * The binding of HMGN to mitotic chromatin is not dependent on a functional HMGN nucleosomal binding domain, and weaker than the binding to interphase nucleosomes in which HMGNs form specific complexes with nucleosomes.[12]
 * The binding of HMGN to mitotic chromatin is not dependent on a functional HMGN nucleosomal binding domain, and weaker than the binding to interphase nucleosomes in which HMGNs form specific complexes with nucleosomes.[12]