H1 Family Histones in the Nucleus

Abstract

H1 histones bind to DNA as they enter and exit the nucleosome. H1 histones have a tripartite structure consisting of a short N-terminal domain, a highly conserved central globular domain, and a lysine-and arginine-rich C-terminal domain. The C-terminal domain comprises approximately half of the total amino acid content of the protein, is essential for the formation of compact chromatin structures, and contains the majority of the amino acid variations that define the individual histone H1 family members. This region contains several cell cycle-regulated phosphorylation sites and is thought to function through a charge-neutralization process, neutralizing the DNA phosphate backbone to allow chromatin compaction. In this study, we use fluorescence microscopy and fluorescence recovery after photobleaching to define the behavior of the individual histone H1 subtypes in vivo. We find that there are dramatic differences in the binding affinity of the individual histone H1 subtypes in vivo and differences in their preference for euchromatin and heterochromatin. Further, we show that subtype-specific properties originate with the C terminus and that the differences in histone H1 binding are not consistent with the relatively small changes in the net charge of the C-terminal domains.The H1 or "linker" histones are a family of very lysine-rich proteins that associate with the stretch of DNA that enters and exits the nucleosome. In the absence of histone H1, the nucleosome comprises ϳ146 bp of DNA that wraps 1.75 turns around the outer surface of the histone octamer (1-3). In the presence of histone H1, the resulting chromatosome contains two complete turns of 168-bp DNA. Histone H1, which is in an ϳ1:1 ratio with the number of nucleosomes in the cell, bends and alters the path of the DNA entering and exiting the nucleosome such that the nucleosome adopts a lollipop-like conformation (4, 5). This change in the trajectory of the linker DNA defines the first step in folding the polynucleosome chain into interphase chromosomes.The discovery of the H1 histones and the identification of its seven variants in the mid 1970s suggested possible roles in development (6). An additional variant, referred to as H1oo, was recently isolated from the oocytes of mice (7). These studies revealed associations between histone H1 subtypes in cell growth and differentiation (8 -12) and in the development of higher eukaryotes (9 -15). In simplistic terms, variants that have been associated with cellular differentiation are the histone H5 and histone H1.0 subtypes, and they are closer in amino acid sequences and most divergent from the other somatic histone H1 variants. Histone H5 is restricted to amphibian and reptile species, where it is found in high abundance in the nucleated but transcriptionally inert erythrocytes (16,17). The nuclei of these mature erythrocytes are almost entirely heterochromatic, allowing them to maintain a very small total volume. In this case, histone H5 expression correlates with the cessation of the RNA polymerase . I...

DNA-induced Secondary Structure of the Carboxyl-terminal Domain of Histone H1

“…The use of the entire carboxyl-terminal domain is important in the study of properties that are not localized to any specific subdomain (27) but rather are determined by the whole domain, such as differential affinity (19) or apoptotic nuclease stimulation (31).…”

“…In mammals, six somatic subtypes (designated H1a-H1e and H1 0 ), a male germ line-specific subtype (H1t), and an oocyte-specific subtype (H1oo) have been identified (11)(12)(13)(14). The subtypes differ in timing of expression (15), extent of phosphorylation (16), turnover rate (17,18), binding affinity (19), and evolutionary stability (20). Differences in DNA condensing capacity have also been demonstrated for some subtypes (21)(22)(23).…”

“…The carboxyl-terminal domain is the primary determinant of histone H1 binding to chromatin in vivo (19,26). Several studies indicate that the ability of linker histones to stabilize chromatin folding resides in the carboxyl-terminal domain of the protein (27)(28)(29).…”

“…The amino-and carboxylterminal domains are highly basic. The distribution of charge in the carboxyl-terminal domain is extremely uniform despite the variation in sequence in the different subtypes (25).The carboxyl-terminal domain is the primary determinant of histone H1 binding to chromatin in vivo (19,26). Several studies indicate that the ability of linker histones to stabilize chromatin folding resides in the carboxyl-terminal domain of the protein (27-29).…”

Testis-specific Linker Histone H1t Is Multiply Phosphorylated during Spermatogenesis

“…In contrast to the COOH-terminal domain, the NH 2 -terminal domain of linker histones is unstructured and does not bind chromatin (57,58). Nevertheless, phosphorylation in this region takes place on several different H1 isoforms from mice (59) and humans (34, 59 -61).…”