Species- and cell-specific expression of H1 histones in tissue culture cells

DNA sequence-dependent positioning of the linker histone in a nucleosome: A single-pair FRET study

Linker histones (LHs) bind to nucleosomes with their globular domain (gH) positioned in either an on- or an off-dyad binding mode. Here, we study the effect of the linker DNA (L-DNA) sequence on the binding of a full-length LH, Xenopus laevis H1.0b, to a Widom 601 nucleosome core particle (NCP) flanked by two 40 bp long L-DNA arms, by single-pair FRET spectroscopy. We varied the sequence of the 11 bp of L-DNA adjoining the NCP on either side, making the sequence either A-tract, purely GC, or mixed with 64% AT. The labeled gH consistently exhibited higher FRET efficiency with the labeled L-DNA containing the A-tract than that with the pure-GC stretch, even when the stretches were swapped. However, it did not exhibit higher FRET efficiency with the L-DNA containing 64% AT-rich mixed DNA when compared to the pure-GC stretch. We explain our observations with a model that shows that the gH binds on dyad and that two arginines mediate recognition of the A-tract via its characteristically narrow minor groove. To investigate whether this on-dyad minor groove-based recognition was distinct from previously identified off-dyad major groove-based recognition, a nucleosome was designed with A-tracts on both the L-DNA arms. One A-tract was complementary to thymine and the other to deoxyuridine. The major groove of the thymine-tract was lined with methyl groups that were absent from the major groove of the deoxyuridine tract. The gH exhibited similar FRET for both these A-tracts, suggesting that it does not interact with the thymine methyl groups exposed on the major groove. Our observations thus complement previous studies that suggest that different LH isoforms may employ different ways of recognizing AT-rich DNA and A-tracts. This adaptability may enable the LH to universally compact scaffold-associated regions and constitutive heterochromatin, which are rich in such sequences.

Characterization of sequence variations in human histone H1.2 and H1.4 subtypes

In humans, eight types of histone H1 exist (H1.1-H1.5, H1 degrees , H1t and H1oo), all consisting of a highly conserved globular domain and less conserved N- and C-terminal tails. Although the precise functions of these isoforms are not yet understood, and H1 subtypes have been found to be dispensable for mammalian development, it is now clear that specific functions may be assigned to certain individual H1 subtypes. Moreover, microsequence variations within the isoforms, such as polymorphisms or mutations, may have biological significance because of the high degree of sequence conservation of these proteins. This study used a hydrophilic interaction liquid chromatographic method to detect sequence variants within the subtypes. Two deviations from wild-type H1 sequences were found. In K562 erythroleukemic cells, alanine at position 17 in H1.2 was replaced by valine, and, in Raji B lymphoblastoid cells, lysine at position 173 in H1.4 was replaced by arginine. We confirmed these findings by DNA sequencing of the corresponding gene segments. In K562 cells, a homozygous GCC-->GTC shift was found at codon 18, giving rise to H1.2 Ala17Val because the initial methionine is removed in H1 histones. Raji cells showed a heterozygous AAA-->AGA codon change at position 174 in H1.4, corresponding to the Lys173Arg substitution. The allele frequency of these sequence variants in a normal Swedish population was found to be 6.8% for the H1.2 GCC-->GTC shift, indicating that this is a relatively frequent polymorphism. The AAA-->AGA codon change in H1.4 was detected only in Raji cells and was not present in a normal population or in six other cell lines derived from individuals suffering from Burkitt's lymphoma. The significance of these sequence variants is unclear, but increasing evidence indicates that minor sequence variations in linker histones may change their binding characteristics, influence chromatin remodeling, and specifically affect important cellular functions.

Identification of histone H1 as a cognate antigen of the ulcerative colitis-associated marker antibody pANCA

Perinuclear anti-neutrophil cytoplasmic antibody (pANCA)(4)is a predominant serum marker of ulcerative colitis (UC), and a familial trait associated with disease susceptibility and disease associated MHC haplotypes. This study characterizes the pANCA antigen defined by representative UC-pANCA human monoclonal antibodies, Fab 5-3 and 5-2. Western blot analysis probed with Fab 5-3 revealed specific binding to a nuclear protein doublet (apparent MW=32-33 kDa) expressed in several cell types. Purification and tryptic peptide sequencing identified the protein as histone H1, and this specificity was confirmed by Fab 5-3 binding to purified H1. Rabbit anti-histone H1 immunostaining and Western blot analysis confirmed that the pANCA epitope is preferentially immunoaccessible in polymorphonuclear neutrophils (PMN). The epitope was localized to the COOH-terminal region by site-specific proteolysis, and recombinant deletants further localized binding activity for both Fab 5-2 and 5-3 to two non-overlapping segments (AA 69-171 and 172-226) associated with a recurring PKKAK motif. Serum IgG binding was detectable to these segments, but was not significantly correlated with pANCA titer or disease status. These findings indicate that histone H1 bears a recurring COOH-terminal epitope recognized by monoclonal ulcerative colitis-associated pANCA marker antibodies, but this epitope is not a predominant specificity of serum pANCA.

Fractionation of human H1 subtypes and characterization of a subtype-specific antibody exhibiting non-uniform nuclear staining

Four histone H1 subtypes and H1(0) were fractionated from human placental nuclei and purified to homogeneity by a combination of Bio-Rex 70 chromatography and reverse-phase high-performance liquid chromatography (RP-HPLC). Polyclonal antibodies were generated in rabbits against one of these subtypes designated H1-3. Antibodies reacted only against this subtype in enzyme-linked immunosorbent assays and Western assays; subtype specificity was documented further by Western blotting of cell and nuclear extracts. They crossreacted with monkey H1, but not with H1 from other vertebrates tested. The epitope(s) recognized were mapped by immunoblotting against peptides prepared by cleavage with N-bromosuccinimide (NBS) and alpha-chymotrypsin; it includes the variant amino-terminal tail of the protein as well as a portion of the globular domain. The antibody stains mitotic chromosomes weakly but uniformly and, unlike antibodies that recognize total H1 which show uniform nuclear staining after indirect immunofluorescence localization, anti-H1-3 exhibits preferential labelling of the nuclear periphery. This non-uniform staining suggests compartmentalization of this subtype which may have functional significance with respect to differential chromatin condensation.

Effect of retinyl acetate on cAMP-dependent protein kinase in transformed mouse 10T1/2 cells

The effect of retinyl acetate (RAC) on the activity of cAMP-dependent protein kinase (PKA) was studied in mouse 10T1/2 cells. The studies revealed that normal 10T1/2 cells had about 13-fold more PKA activity than did methylcholanthrene-transformed cells (MCA cells). The addition of RAC to MCA cells increased the activity of PKA about 3-fold as measured by the in vitro phosphorylation of a specific site in H1 histone (site A) or Kemptide. The increased PKA activity coincided with a reduction in the rate of cell replication of MCA cells, about 24 hr after exposure to the retinoid. Addition of forskolin to RAC-treated MCA cells resulted in a further reduction in the rate of cell replication, and this suggested that the enhanced PKA activity was also capable of action in vivo. To test this notion, MCA cells were grown with and without RAC, and the phosphorylation of the H1 histone at site A, a site known to be phosphorylated by PKA in cells treated with hormones or other agonists which activate PKA, was studied in vivo. RAC, by itself, was capable of causing an increase in the phosphorylation of the H1 histone at site A, demonstrating that the retinoid-mediated increase in PKA activity was sufficient to cause the enhanced phosphorylation of a known substrate.

H1 histone synthesis and phosphorylation in mouse mammary gland in vitro

The synthesis and phosphorylation of H1 histones were studied in organ cultures of midpregnant mouse mammary glands exposed to various combinations of insulin, cortisol and prolactin over a 48-h period. The synthesis of specific H1 histone subtypes was changed only when all three hormones were present, and the effect was most pronounced during the first 24 h of culture, a period of cell replication. The 3-hormone combination also stimulated the phosphorylation of the N-terminal region of the H1 histone, and this also occurred maximally during the first 24 h of culture. The enhanced phosphorylation of the N-terminal region of the H1 histone included a site sensitive to phosphorylation by cyclic AMP-dependent protein kinase. Thus, hormones which stimulate mammary development in vitro influence the synthesis and specific phosphorylation of H1 histones during a period of cell replication preceding the expression of milk protein genes.

A study on the heterogeneity of histone H1 from dry maize embryos

Maize dry embryo cells have been chosen as model for the study of H1 histone complement of metabolically inactive plant chromatin. H1 has been fractionated into two distinct bands in acetic acid/urea gels. Analysis of each of these bands in SDS-containing gels has shown that maize embryo H1 is heterogeneous, consisting of at least six proteins. Cross-reactivity of maize H1 species with antibodies against mouse liver total H1 and against its individual variants indicates that they share common immunological determinants but differ substantially from each other when compared by peptide mapping. It is concluded that a couple of the plant H1 subfractions are related to animal H1A histone and that another one is related to H1B histone. The other three maize H1 variants are closely related to each other and they also share common immunological determinants with mouse H1A, and possibly with H1B. All maize H1 species slightly cross-react with antibodies against H1 degree, suggesting that no one of the plant subfractions could be characterized as H1 degree in particular. One of the proteins co-extracted and comigrating with H1 is most probably an embryo storage protein.

Effect of exogenous histone H5 on integration of histone H1 in rat liver chromatin. Correlations with aberrant epsilon-N-methylation of histone H1

Binding modes of histones H1 and H5, and their competition for chromatin-binding sites in rat liver nuclei, were correlated with aberrant N-methylation of H1 histone lysine residues, induced by chicken erythrocyte histone H5, in order to gain more insight into the integration of lysine-rich histones in chromatin. Addition of approx. 2.5 molecules of histone H5 per nucleosome to rat liver nuclei increases the ratio of total basic residues in histones to DNA nucleotides (BR/NT) in the nuclear chromatin from 1.0 to 1.5. At this concentration, approx. 0.7 molecule of histone H5 is bound per nucleosome, and there is no displacement of histone H1 from the nuclear chromatin. If S-adenosyl[Me-3H]methionine is present in the incubation mixture, the aberrant incorporation of labeled methyl groups into histone H1 reaches a maximum at this concentration of histone H5. The radioactivity present in histone H1 from nuclei incubated with labeled AdoMet at a total BR/NT ratio of 1.5: resides mainly in a histone H1 subfraction tentatively identified by Bio-Rex 70 chromatography and acrylamide gel electrophoresis as histone H1c; presents as a single spot upon peptide mapping of tryptic hydrolysates by means of two-dimensional thin-layer chromatography; and elutes in the position of mono-N-methyllysine upon ion-exchange chromatography of histone H1 hydrolysates. Upon further increase of the BR/NT ratio, the following changes are produced: a gradual decrease in radioactive methyl uptake into histone H1; a gradual displacement of histone H1 from the chromatin; increased binding of histone H5 in chromatin, up to a maximum of 3.4 residues per nucleosome; and a slowly increasing uptake of label into histone H5. The combined data from histone H1/H5 binding and histone H1 methylation studies suggest that upon addition of exogenous histone H5 to rat liver nuclei the binding of two lysine-rich histones per nucleosome plays a significant role in the induction of specific changes in chromatin structure, which in vivo may have important functional implications in terms of chromatin condensation and suppression of transcription.

Changes in the H-1 histone complement during myogenesis. I. Establishment by differential coupling of H-1 species synthesis to DNA replication

Proportions of the four major chicken H-1 histones (referred to as H-1's a-d) change during in vitro skeletal myogenesis. As myoblasts fuse and differentiate into myotubes, the relative amount of H-1c increases dramatically. The change occurs primarily because synthesis of the H-1 species is coupled to DNA synthesis to different extents. H-1c synthesis is least tightly coupled to DNA replication in precursor myoblasts and in differentiated myotubes. Thus H-1c synthesis predominates after dividing myoblasts fuse into postmitotic myotubes. This results in the replacement of pre-existing H-1 and therefore increases the relative amount of H-1c. Differences in the stability of the H-1's are also involved in changing H-1 proportions. The results show that changes in H-1 proportions during myogenesis are a consequence of withdrawal from the cell cycle. The data provides a general mechanistic explanation of how tissue-specific H-1 proportions are established.

Isolation and characterization of five subfractions of chicken erythrocyte histone H1

Chicken erythrocyte chromatin contains, besides the specific histone H5, a set of histone H1 subtypes. Five of them were isolated by ion-exchange chromatography and these very related proteins, called H1A, H1B, H1C, H1D and H1E, were characterized by their amino acid compositions.

Molecular and cell isoforms during development

Development proceeds by way of a discrete yet overlapping series of biosynthetic and restructuring events that result in the continued molding of tissues and organs into highly restricted and specialized states required for adult function. Individual molecules and cells are replaced by molecular and cellular variants, called isoforms; these arise and function during embryonic development or later life. Isoforms, whether molecular or cellular, have been identified by their structural differences, which allow separation and characterization of each variant. These isoforms play a central and controlling role in the continued and dynamic remodeling that takes place during development. Descriptions of the individual phases of the orderly replacement of one isoform for another provides an experimental context in which the process of development can be better understood.

Occurrence of histone H10-related protein fraction in trout liver

The lysine-rich histones of trout liver were studied in order to see whether a protein similar to mammalian histone H1(0) exists in this lower vertebrate. The biochemical methods used involved SDS and acid-urea polyacrylamide gel electrophoresis, gel exclusion and ion-exchange chromatography. Specific rabbit antibodies were elicited against chromatographically-purified mouse liver H1(0) and applied for further characterization of the presumptive trout-liver H1(0) fraction. The results from the indirect immunofluorescence on nitrocellulose-immobilized crude H1 subfractions and from the complement fixation test supplied additional evidence for the existence of an H1(0)-like protein in trout liver.