Histone proteins have central roles in both chromatin organization (as structural units of the nucleosome) and gene regulation (as dynamic components that have a direct impact on DNA transcription and replication). Eukaryotic DNA wraps around a histone octamer to form a nucleosome, the first order of compaction of eukaryotic chromatin. The core histone octamer is composed of a central H3-H4 tetramer and two flanking H2A-H2B dimers. Each of the core histone 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. 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 characterized by high sequence homology among the family of linker 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 (see <PDB:1HST>) [1,2,3,4,E1].

Some proteins known to contain a H15 domain are listed below:

• Eukaryotic histone H1. The histones H1 constitute a family with many variants, differing in their affinity for chromatin. Several variants are simultaneously present in a single cell. For example, the nucleated erythrocytes of birds contain both H1 and H5, the latter being an extreme variant of H1.
• Eukaryotic MHYST family of histone acetyltransferase. Histone acetyltransferases transfer an acetyl group from acetyl-CoA to the epsylon- amino group of lysine within the basic NH2-termini of histones, which bind the acidic phosphates of DNA [5].

The profile we developed covers the entire H15 domain.