A compendium of the histone H1 family of somatic subtypes: An elusive cast of characters and their characteristics

Abstract

The last 35 years has seen a substantial amount of information collected about the somatic H1 subtypes, yet much of this work has been overshadowed by research into highly divergent isoforms of H1, such as H5. Reports from several laboratories in the past few years have begun to call into question some of the traditional views regarding the general function of linker histones and their heterogeneity. Hence, the impression in some circles is that less is known about these ubiquitous nuclear proteins as compared with the core histones. The goal of the following review is to acquaint the reader with the ubiquitous somatic Hls by categorizing them and their characteristics into several classes. The reasons for our current state of misunderstanding is put into a historical context along with recent controversies centering on the role of H1 in the nucleus. Finally, we propose a model that may explain the functional role of H1 heterogeneity in chromatin compaction.

Role of linker histone in chromatin structure and function: H1 stoichiometry and nucleosome repeat length

“…The mouse has 6 somatic H1 variants (Parseghian & Hamkalo 2001) as well as testis-and oocyte-specific subtypes, and the systematic knockout of up to three somatic variants has provided animals and embryonic stem (ES) cells with a range of total H1 levels (Fan et al 2003(Fan et al , 2005. For each cell type, the content of each H1 subtype was quantitated by HPLC, and NRL values were determined from micrococcal nuclease digests.…”

Expression of human oocyte-specific linker histone protein and its incorporation into sperm chromatin during fertilization

“…4): [1] no immunoreactivity for the hH1FOO antibody and persistence of condensed chromatin (15 oocytes, 45%), [2] immunoreactivity for hH1FOO with persistence of condensed chromatin (3 oocytes, 9%), and [3] immunoreactivity for hH1FOO with decondensed chromatin (15 oocytes 45%). Interestingly, none of the oocytes contained sperm that showed decondensation but were nonreactive to the hH1foo antibody.…”

“…A previous cytogenetic study of unfertilized human oocytes after ICSI revealed various outcomes of the injected sperm: [1] the entire sperm was missing or was ejected outside the ooplasm, [2] sperm chromatin was undecondensed, or [3] there was variable premature chromosome condensation (27). Among these outcomes, failure of decondensation may arise from incomplete immobilization of sperm due to insufficient damage to the sperm plasma membrane.…”

“…A nucleosome has a protein core, which is an octamer containing two molecules each of histones H2A, H2B, H3, and H4, with DNA wound around its surface. It is believed that histone H1, also termed a linker histone, binds to the linker DNA and plays an important role in the development and stabilization of higher-order chromatin structure and also possibly is involved in regulating gene expression (1)(2)(3)(4).…”

Purification and characterization of histone H1 variants from human placenta

“…H1 histones exist in multiple forms and have similar amino acid compositions but are hypothesized to have very distinct functions [2,3]. The sequences encoding these genes show a high degree of gene polymorphism; in mammals, these polymorphisms result in five to eight subtypes with distinct biological functions.…”