Human linker histones: interplay between phosphorylation and O-β-GlcNAc to mediate chromatin structural modifications

Eukaryotic genome is packaged into a structure known as chromatin. The basic structural unit of chromatin called as nucleosome is composed of DNA and proteins [1]. The major proteins involved in chromatin structure are histone proteins. Histone proteins are of five types: H1, H2A, H2B, H3 and H4 [2-4]. Histone H1 is known as linker histone while the other four histone proteins are collectively known as core histones. This DNA-protein complex is the tempelate for a number of essential cell processes including transcription recombination, repair and replication. Histone H1 is located on the linker DNA that goes between the nucleosomes in chromatin structure [5]. Linker DNA which is associated with linker histone H1 interconnects core particles, varies in length, depending on species and tissue [6]. Organization of DNA into nucleosomes by histone proteins and folding of nucleosomes into higher-order chromatin structure is generally believed to compact DNA and make it inaccessible to transcription factors [7]. Linker histones H1 are necessary for modulating chromatin structure and function at multiple levels [8].Organisms contain a variety of subtypes of linker histone which exhibit significant sequence divergence and distinct patterns of expression differentiation and development [9]. The H1 linker histones are the most divergent group. Usually nine subtypes of linker histone H1 are present in mammals including H1.1, H1.2, H1.3, H1.4, H1.5, H1o, H1Foo, H1.t [10] and H1.x [11]. Linker histone sub-types are classified according to their tightly regulated expression pattern during embronyal development and cell differentiation [12]. All known sub-types of linker histone contain a common domain structure. Linker histones consist of a short N-terminal, a highly conserved central globular domain and a long C-terminal domain [13]. Somatic cells contain almost all sub-types of linker histone H1 [12]. In vitro, H1-containing chromatin shows strong inhibition of transcription [1