1/15/2024 0 Comments Scaffold protein h1![]() ![]() The HMG terminology is coined due to their unusual solubility, along with smaller size and migration properties in comparison to chromatin proteins during gel electrophoresis. HMG protein superfamilies are one of the major groups of architectural protein present in the eukaryotic cells. One such modifier is the High Mobility Group (HMG) class of proteins which are well known for their roles as architectural DNA binder in the nucleus and mitochondria, as signaling regulators in the cytoplasm and as inflammatory cytokines in the extracellular space. These DNA binding proteins that help to stabilize the DNA structure within the cell form the ‘Architectural proteins’ that are grouped as wrappers, benders and bridgers. Thus, DNA packaging begins with supercoiling and then aided by macromolecular crowding which in turn is stabilized by DNA binding proteins such as histones. This disruption can be of two types: (1) transient unwrapping of DNA at certain position followed by re-wrapping at the same entry/exit point and (2) migration of the nucleosome complex along the DNA involving simultaneous unwrapping and reestablishment of new DNA protein contact with a new sequence. However, disruption of the local and higher order nucleosome structure is the primary requisite for proper access of DNA sequences by different nuclear machinery. Importance of H1 is further accentuated by the fact that loss of H1 results in change in gene expression by regulating the access of transcriptional regulators. But binding to DNA is not solely determined by the distribution of positive charged residue at the C-terminal of histone H1, rather specific residues play an essential role in maintaining the specificity of the interaction. The highly basic C-terminal domain of H1 protein actually interacts with the negatively charged linker DNA and facilitates the chromatin compaction. Although the exact organization of H1 is not known, but it is well established that H1 globular domain contacts the DNA near the nucleosome dyad axis and the adjacent linker DNA, thus stabilizes the wrapped DNA-histone octamer structure. The stability of the higher order chromatin structure needs the involvement of histone H1 that binds the linker DNA between two nucleosomes. The arrays of nucleosomes that constitute “beads on a string arrangement” structure are further packaged into the 30 nm fibre by coiling around itself. Primarily, this packaging starts with wrapping of DNA strand around the histone octamer involving 1.7 turns encompassing 147 bps. This versatile packaging of DNA makes the architecture of the genome extremely fascinating and one of the most studied aspect of genome biology in the last few decades. In eukaryotes, nuclear DNA are packaged tightly to be accommodated in the small nucleus which is accessible to the DNA dependent processes and at the same time space effective. These nuclear elements not only act as architectural proteins but also play a multifaceted role in chromatin dynamics by facilitating interaction with nucleosomes, nucleosome-remodeling machines, transcription factors and histones. This review focuses on the structure–function relationship of three broad families of High mobility groups (HMGs) of protein, namely HMGA, HMGN and HMG-Box which are major chromatin architectural components of the eukaryotes. The regulation of these proteins and their interaction with DNA modify the cellular phenotype by the modulation of gene expression. Architectural proteins are instrumental in organizing the dynamic higher order chromatin structure by effectuating a concerted effort among themselves and other nuclear proteins across spatio-temporal scales. Genomic DNA needs to be packaged such that it takes minimum space and can simultaneously be accessed for various DNA dependent processes. DNA super-coiling and architectural proteins are the key players that maintain the chromatin in its compact state. ![]()
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