High mobility group (HMG) proteins are the second most abundant family of chromosomal proteins. HMG proteins are proteins with an HMG-box domain that binds kinked or unwound DNA in a sequence non-specific way with high affinity. The HMG-box structure contains three alpha-helices separated by loops. HMG proteins contain two DNA-binding HMGbox domains that show structural and functional differences, as well as a long, acidic C-terminal domain rich in aspartic and glutamic acid residues (Sebastian et al., 2009, Lange et al., 2009).

The HMG-box domain can be found in a wide variety of proteins serving many biological purposes, such as DNA repair, transcription, and recombination. Nucleosome disruption plays a key role in such nuclear processes including transcription, DNA repair and recombination. Atomic force microscopy (AFM) and optical tweezers (OT) experiments were employed to show that high mobility group B (HMGB) proteins strongly disrupt nucleosomes, revealing a new mechanism for regulation of chromatin accessibility. We find that both the double box yeast Hmo1 and the single box yeast Nhp6A display strong binding preferences for nucleosomes over linker DNA, and both HMGB proteins destabilize and unwind DNA from the H2A-H2B dimers. However, unlike Nhp6A, Hmo1 also releases half of the DNA held by the (H3-H4)2 tetramer. This difference in nucleosome destabilization may explain why Nhp6A and Hmo1 function at different genomic sites. Hmo1 is enriched at highly transcribed ribosomal genes, known to be depleted of histones. In contrast, Nhp6A is found across euchromatin, pointing to a significant difference in cellular function (McCauley et al., 2019).

In rice, the expression of OsHMGB707 was up-regulated by dehydration and high salt treatment. Its overexpression significantly enhanced drought tolerance in transgenic rice plants, whereas its knockdown through RNA interference (RNAi) did not affect the drought tolerance of the transgenic rice plants. Notably, OsHMGB707-GFP is localized in the cell nucleus, and OsHMGB707 is protein-bound to the synthetic four-way junction DNA (Xu et al., 2021).

HMG proteins have been studied in maize and it was found that specific nucleosome binding of the plant HMGB proteins requires simultaneous DNA and histone contacts. (Lichota et al., 2003). Ectopic expression of the maize chromosomal HMGB1 protein caused defects in root development of tobacco seedlings (Lichota et al., 2004). Proteins that contain both an ARID and an HMG-box domain, are termed ARID-HMG proteins, appear to be specific for plants. This protein family is conserved in higher plants (both mono- and dicot plants) as well as lower plants such as the moss Physcomitrella.  The genes encoding ARID-HMG1 and ARID-HMG2 are widely expressed in Arabidopsis but at different levels. Subcellular localization experiments studying ARID-HMG1 and ARID-HMG2 fused to GFP by fluorescence microscopy show that both proteins localize primarily to cell nuclei. Analyses of the DNA-binding properties using electrophoretic mobility shift assays revealed that mediated by the HMG-box domain, ARID-HMG1 binds structure specifically to DNA minicircles. Mediated by the ARID, the protein binds preferentially to A/T-rich DNA, when compared with G/C-rich DNA. Therefore, both DNA-binding domains contribute to the DNA interactions of ARID-HMG1. Accordingly, the protein combines DNA-binding properties characteristic of ARID and HMG-box proteins (Hansen et al., 2008).

Last updated June 2023 by John Gray

References:

Sebastian NT, Bystry EM, Becker NA, Maher LJ 3rd. Enhancement of DNA flexibility in vitro and in vivo by HMGB box A proteins carrying box B residues. Biochemistry. 2009 Mar 17;48(10):2125-34. doi: 10.1021/bi802269f. PMID: 19236006; PMCID: PMC2668924.

Lange SS, Vasquez KM. HMGB1: the jack-of-all-trades protein is a master DNA repair mechanic. Mol Carcinog. 2009 Jul;48(7):571-80. doi: 10.1002/mc.20544. PMID: 19360789; PMCID: PMC2856944.

McCauley MJ, Huo R, Becker N, Holte MN, Muthurajan UM, Rouzina I, Luger K, Maher LJ 3rd, Israeloff NE, Williams MC. Single and double box HMGB proteins differentially destabilize nucleosomes. Nucleic Acids Res. 2019 Jan 25;47(2):666-678. doi: 10.1093/nar/gky1119. PMID: 30445475; PMCID: PMC6344895.

Xu K, Chen S, Li T, Yu S, Zhao H, Liu H, Luo L. Overexpression of OsHMGB707, a High Mobility Group Protein, Enhances Rice Drought Tolerance by Promoting Stress-Related Gene Expression. Front Plant Sci. 2021 Aug 5;12:711271. doi: 10.3389/fpls.2021.711271. PMID: 34421959; PMCID: PMC8375505.

Lichota J, Grasser KD. Interaction of maize chromatin-associated HMG proteins with mononucleosomes: role of core and linker histones. Biol Chem. 2003 Jul;384(7):1019-27. doi: 10.1515/BC.2003.114. PMID: 12956418.

Lichota J, Ritt C, Grasser KD. Ectopic expression of the maize chromosomal HMGB1 protein causes defects in root development of tobacco seedlings. Biochem Biophys Res Commun. 2004 May 21;318(1):317-22. doi: 10.1016/j.bbrc.2004.03.193. PMID: 15110790.

Hansen FT, Madsen CK, Nordland AM, Grasser M, Merkle T, Grasser KD. A novel family of plant DNA-binding proteins containing both HMG-box and AT-rich interaction domains. Biochemistry. 2008 Dec 16;47(50):13207-14. doi: 10.1021/bi801772k. PMID: 19053246.