FC0027
H1⁰ linker histone  -  DFF40/CAD apoptotic nuclease

Biological function
H1⁰ linker histones affect linker DNA structures and play a role in stabilization of higher-order chromatin assemblies via their disordered 100 AA C-terminal domain. The H1 CTD also has been shown to mediate protein-protein interactions involved in H1-dependent activation of the apoptotic nuclease, DFF40/ CAD. Linker histones in general are multifunctional, with roles in chromatin condensation nucleosome spacing, specific gene expression DNA methylation and other nuclear processes. In addition to chromatin, linker histones bind to many nuclear proteins, e.g., DFF40/CAD, BAF, Purα/YB1.

Domain organization/sequence features
Linker histones are modularly structured proteins that have an ~35 residue unstructured N-terminal domain (NTD), a central globular winged helix domain, and an ~100 residue unstructured C-terminal domain (CTD). Linker histones bind to chromatin fibers through interaction of the globular domain with nucleosomal sites(s), and the CTD with linker DNA. Higher eukaryotes have at least six somatic linker histone isoforms, which differ primarily in their CTD primary sequences. The H1 isoform CTDs do, however, share a very similar and characteristic amino acid composition.

Structural evidence
CTD is absent from crystal structure of the nucleosome, indicating that it is partly disordered in the bound state.

Biochemical evidence
The CTD region that binds to the enzyme is large and partially overlaps with the two CTD regions that bind linker DNA and stabilize condensed chromatin. Interestingly, all somatic linker histone isoforms activated the enzyme identically in vitro. Moreover, all free CTD peptides that were at least 47 residues in length could bind to and activate the enzyme, regardless of their primary sequence and original location in the intact CTD. Thus, amino acid composition and location of the CTD region relative to the winged helix domain also appear to be the determinants of CTD-protein interactions. Detailed studies show that randomization of sequence not, but shifting composition affects function of the H1 CTD.

Structure/Mechanism
Owing to the high positive charge of H1 CTD, it was thought to function through an electrostatic mechanism. Detailed mutagenesis and scrambling experiments however showed that only two discontinuous stretches of CTD (98–122 and 147– 170) mediate interactions with DNA and facilitate chromatin fiber condensation irrespective of their exact order and primary sequence. Stabilization of chromatin structure was dependent only on the amino acid composition of these two regions, which is conserved in all H1 isoforms and their distance from the globular domain.

Mechanism category
tethering

Significance
Fuzziness enables interactions with different partners via different pattern of α-helices. mediate interactions with linker DNA as well as with nuclear proteins. Condensation of the malleable chromatin fiber is induced by a specific amino acid composition (encoding disordered protein segments) in contrast to specific sequence requirements.

Submitted by
Jeffrey C Hansen   jchansen@lamar.colostate.edu