Protein Domain : Linker histone H1/H5 IPR005819

Type	Family
Description	Histone proteins have central roles in both chromatin organisation (as structural units of the nucleosome) and gene regulation (as dynamic componentsthat have a direct impact on DNA transcription and replication). Eukaryotic DNA wraps around a histone octamer to form a nucleosome, the first order ofcompaction of eukaryotic chromatin. The core histone octamer is composed of a central H3-H4 tetramer and two flanking H2A-H2B dimers. Each of the corehistone contains a common structural motif, called the histone fold, which facilitates the interactions between the individual core histones.In addition to the core histones, there is a "linker histone"called H1 (or H5 in avian species). The linker histones present in all multicellular eukaryotes are the most divergent group of histones, with numerous cell type- and stage-specific variant. Linker histone H1 is an essential component of chromatin structure. H1 links nucleosomes into higher order structures. Histone H5 performs the same function as histone H1, and replaces H1 in certain cells. The structure of GH5, the globular domain of the linker histone H5 is known [, ]. The fold is similar to the DNA-binding domain of the catabolite gene activator protein, CAP, thus providing a possible model for the binding of GH5 to DNA.The linker histones, which do not contain the histone fold motif, are critical to the higher-order compaction of chromatin, because they bind to internucleosomal DNA and facilitate interactions between individual nucleosomes. In addition, H1 variants have been shown to be involved in the regulation of developmental genes. A common feature of this protein family is a tripartite structure in which a globular (H15) domain of about 80 amino acids is flanked by two less structured N- and C-terminal tails. The H15 domain is also characterised by high sequence homology among the family oflinker histones. The highly conserved H15 domain is essential for the binding of H1 or H5 to the nucleosome. It consists of a three helix bundle (I-III),with a β-hairpin at the C terminus. There is also a short three-residue stretch between helices I and II that is in the β-strand conformation.Together with the C-terminal β-hairpin, this strand forms the third strand of an antiparallel β-sheet [, , , ].Histone H5 is a nuclear protein involved in the condensation of nucleosome chains into higher order structures. In this respect, it performs the same function as histone H1, and replaces H1 in certain cells. The structure of GH5, the globular domain (residues 22-100) of the linker histone H5, has been solved. The fold is similar to the DNA-binding domain of the catabolite gene activator protein, CAP, thus providing a possible model for the binding of GH5 to DNA. The structure comprises 3 α-helices and 2 short β-strands [ , ].
Short Name	H1/H5