The complete amino-acid sequence of histone H2B from erythrocytes of the adult domestic fowl Gallus domesticus

Epitope recognition in histone H1 by SLE autoantibodies in the presence of a DNA-ligand

To investigate the specificity of anti H1 antibodies peptides from the N- and C-domain of H1 and the synthetic oligonucleotide (AT)6 were complexed. Circular dichroism (CD) spectroscopy indicated that the free peptides H1(1-16), H1(204-218) and C(121-210) in low salt buffer assume a random structure but become helical when bound to the oligonucleotide. The structured and unstructured H1 fragments were then analyzed by enzyme linked immunosorbent assay (ELISA) with anti-H1 antibodies in sera from patients with systemic lupus erythematosis (SLE) and with the monoclonal anti-H1 antibody MRA-12 derived from MLR lpr/lpr autoimmune mice. Binding of these antibodies to H1(204-218) and C was inhibited to a level of 50% when these H1 peptides were complexed with (AT)6. When the same antibody was tested with H1 fragment GC(34-210), attachment to oligonucleotide (AT)6 did not influence antibody binding. Competition studies with liquid phase GC and C antigen against solid phase GC and C indicated that liquid phase GC was more efficient in displacing antibody binding reactivity than liquid phase C. The displacement effect of both liquid phase antigens was greatest against solid phase C. We conclude that anti-H1 autoantibodies are directed against an epitope located near the junction of the G- and C-domain which is exposed and not masked when H1 is bound to DNA.

Core histone-DNA interactions in sea urchin sperm chromatin. The N-terminal tail of H2B interacts with linker DNA

A three-stage chemical modification procedure [Lambert, S. F. & Thomas, J. O. (1986) Eur. J. Biochem. 160, 191-201; Thomas, J. O. & Wilson, C. M. (1986) EMBO J. 5, 3531-3537] for selectively radiolabelling lysine residues that interact with DNA has been used to investigate core histone--DNA interactions in sea urchin sperm chromatin, in particular to determine the binding site of the long N-terminal domain of sperm-specific H2B. Comparison of the patterns of radiolabelling of core histones from extended chromatin and nucleosome core particles (which lack linker DNA) reveals the regions of the histones involved in interactions with the linker. The results show that the N-terminal domain of H2B is bound to DNA outside the 146-bp nucleosome core, presumably to the linker DNA. H2A and H4 make no substantial contacts with the linker in extended chromatin; the N-terminal tail of H4 is bound within the core particle, but the N-terminal tail of H2A is not bound in core particles or in extended chromatin, and may therefore have a role in higher-order structure. H3, like H2B, makes contacts with DNA outside the 146-bp nucleosome core in its N-terminal region, as well as elsewhere, and probably interacts with the two 10-bp extensions that complete the two turns of DNA in the nucleosome and/or with the linker.

Histone accessibility determined by lysine-specific acetylation in chicken erythrocyte nuclei

N-Hydroxysulfosuccinimidyl [3H]acetate was synthesized and, following the determination of the optimal reaction conditions, was used to acetylate histones in chicken erythrocyte nuclei at 4 degrees C, pH 8. The histones were extracted from the labelled nuclei and the distribution of the acetyl groups determined from the amount of tritiated acetate in isolated peptides. The relative degree of acetylation of molecules was H1 1.0, H5 0.81, H2B 0.48, H2A 0.24, H3 0.24, H4 0.16. Histone H1 is the most exposed histone followed by H5. The core histones are much less accessible to chemical modification than the linker histones by a factor of 4-5. Histones H2A, H2B and H5 appear to be labelled at random along the entire polypeptide chain, while histones H3 and H4 are labelled almost exclusively in the first 30 residues from the N terminus. Control and acetylated chicken erythrocyte nuclei were digested with DNase I and the resulting DNA hybridized to globin and ovalbumin cDNAs. Acetylation, at 14 molecules acetate/core nucleosome or 20 molecules acetate/chromatosome, increased the DNase I sensitivity of the ovalbumin gene to that of the globin sequences in the control sample, while the globin sequences became even more nuclease-sensitive. Our results suggest that increased sensitivity of chromatin towards nuclease digestion might be due to increased solubility of the chromatin fibre.

Autoantigenic histone epitopes: a comparison between procainamide- and hydralazine-induced lupus

Using the technique of immunoblotting, we assessed the ability of sera from 19 patients with drug-induced lupus to bind individual histones and specific histone fragments. The pattern of histone epitopes bound by sera from 9 patients with procainamide-induced lupus was very similar to that described previously in spontaneous systemic lupus erythematosus. In contrast, sera from 10 patients with hydralazine-induced lupus bound a broader array of individual histones and recognized a different set of histone epitopes. We conclude that these 2 drugs induce antihistone antibodies through somewhat different mechanisms, which possibly involve differences in their ability to structurally alter chromatin.

Structure and organization of the chicken H2B histone gene family

The results of Southern blotting experiments confirm that the chicken H2B histone gene family contains eight highly homologous members. One or two more sequences which are considerably divergent from the others appear to exist in the chicken genome. Seven of the eight H2B genes have been cloned and sequenced. All seven genes fall in two histone gene clusters, but no common arrangement exists for the clusters themselves. Three different H2B protein variants are encoded by these seven genes. The nucleotide sequence homology among the genes within their coding sequences appears to exceed that required for the corresponding protein sequences, suggesting that histone H2B mRNA sequence and structure are both selected during evolution. An analysis of the 5' flanking sequence data reveals that these genes possess CCAAT and TATA boxes, elements commonly associated with genes transcribed by RNA polymerase II. In addition, these genes all share an H2B-specific element of the form: ATTTGCATA. The 3' sequences of these genes contain the hyphenated symmetrical dyad homology and downstream purine-rich sequence shared by histone genes in general.

Predictability of sequence homologies among lysine-rich histones by immunological distance

The relationship between immunological distance (I.D.) measured by microcomplement fixation and amino acid sequence difference for lysine-rich histones was tested using antisera to lysine-rich histones of known sequence, chicken H1 and H5, goose H5, and trout H1 as well as to trout H5. The best relationship between I.D. (y) and percent sequence difference (x) for lysine-rich histones, y = 2x, applies as well to other histones of known sequence but it differs from y = 5x, reported for other proteins and often used for histones. Although deviations indicate that I.D. is a poor predictor of primary sequence differences among histones, it suggests that trout H5 is more closely related to H1 than to chicken H5.

Lysine-containing DNA-binding regions on the surface of the histone octamer in the nucleosome core particle

The DNA bound on the surface of the histone octamer in the nucleosome core particle partially protects the epsilon-amino side-chains of a subset of the lysine residues from reductive methylation. Most of the strongly protected lysines, which probably define the path of the DNA on the octamer surface, are in the globular ('structured') regions of the core histones rather than in the N-terminal or C-terminal 'tails'. Analysis of the protected peptides shows that the three strongest lysine-containing DNA-binding sites in the core histones contain the sequence-Lys/Arg-Lys-Thr/Ser-. On the assumption that the lysine-containing regions protected from chemical modification are also those found in lysine-DNA cross-links in another study [Mirzabekov et al. (1978) Proc. Natl Acad. Sci. USA 75, 4184-4188], particular DNA-protected peptides may be tentatively assigned to particular DNA contact points. This leads to a more detailed description of the DNA-binding regions on the octamer surface in the nucleosome core particle. Strong contacts, reflected in strongly protected lysines, may well contribute to the distortion of the DNA from smooth bending [Richmond et al. (1984) Nature (Lond.) 311, 532-537].

Primary structure of histone H2B from gonads of the starfish Asterias rubens. Identification of an N-dimethylproline residue at the amino-terminal

The complete amino acid sequence (121 residues) of histone H2B from gonads of the starfish Asterias rubens has been established from structural data obtained essentially from large fragments generated by cleavage of histone H2B at aspartyl residues and by limited hydrolysis of the dimer H2A-H2B with mouse submaxillary gland protease. No real sequence homology can be found between the amino-terminal sequence (residues 1-21) of starfish and calf H2B. One non-conservative substitution (serine-32 in calf----lysine-28 in starfish) leads to the presence of a cluster of eight basic residues (sequence 23-30) and to the disappearance of a potential site of phosphorylation. A particular structural feature of starfish histone H2B is the presence of N-dimethylproline at its amino-terminal end. By comparison with N-terminal acetylation, which is commonly found in histones, N-terminal methylation is rarely observed. At the present time the functional significance of the N-terminal methylation as well as that of the proline-rich nature of the amino-terminal sequence of the starfish histone H2B remain to be defined.

Antigenic structure of histone H2B

Antigenic determinants of histone H2B were localized using a series of 23 overlapping fragments of H2B obtained either by chemical and enzymatic cleavage of the histone or by solid-phase peptide synthesis. The ability of peptides to bind H2B antibodies was measured in an enzyme-linked immunosorbent assay, using antisera directed against calf thymus and chicken erythrocyte H2B as well as four anti H2B monoclonal antibodies obtained from autoimmune mice. Seven antigenic determinants were localized in the H2B molecule in the vicinity of residues 1-11, 6-18, 15-25, 26-35, 50-65, 94-113 and 114-125. Two of these determinants (residues 6-18 and 26-35) were revealed only through the binding properties of antibodies isolated from autoimmune mice. The usual correlation between hydrophilicity and antigenicity was found to hold for four of the epitopes, and the N- and C-termini of H2B were both antigenically active.

The major core histone antigenic determinants in systemic lupus erythematosus are in the trypsin-sensitive regions

Antibody responses against nucleosome core histones in systemic lupus erythematosus have been shown, by immunoblotting, to be directed largely against the trypsin-sensitive regions of the histones. These occur at the N-terminal regions of all 4 core histones and at the C-terminal ends of H2A and H3. Since these regions are often not the most antigenic when individual histones are used as immunogens, and appear to be exposed in the nucleosome, the active immunogens in systemic lupus erythematosus seem likely to be chromatin-bound, rather than free, histones.

Neutron scattering studies of the H2a-H2b and (H3-H4)2 histone complexes

Neutron scattering experiments have shown that both the (H3-H4)2 and H2a-H2b histone complexes are quite asymmetric in solution. The (H3-H4)2 tetramer is an oblate or flattened structure, with a radius of gyration almost as large as that of the core octamer. If the tetramer is primarily globular, it must have an axial ratio of about 1:5. It is more likely, however, that this asymmetry results in part from N-terminal arms that extend outward approximately within the major plane of the particle. If this is the case, less asymmetric models for the globular part of the tetramer, including a dislocated disk of the type proposed by Klug et al. (23), can be made consistent with the scattering data. The H2a-H2b dimer, on the other hand, is an elongated structure. The low resolution data are in good agreement with those calculated for a cylindrical model 64 X 27 A, but other elongated models fit those data almost as well, including one that approximates free N-terminal arms at each end. Free arms are not necessary, but they must extend from the ends if they exist. A contrast matching experiment done with 50% deuterated H2b and undeuterated H2a in the reconstituted dimer showed that these two histones must each be rather elongated within the complex and are not just confined to one end. The amount of scattering contrast between the undeuterated and 50% deuterated histones was sufficient to suggest further experiments using complexes reconstituted from mixtures of undeuterated and partially deuterated histones which will help elucidate their arrangement within the histone complexes and within the octamer core of the nucleosome core particle.

Antibodies to histones in systemic lupus erythematosus: localization of prominent autoantigens on histones H1 and H2B

By the technique of immunoblotting we have assessed the ability of sera from 24 patients with systemic lupus erythematosus to bind nuclear proteins. Of the 11 patients who had antibodies to histones, 10 had antibodies to histone H1 and 9 of these also had antibodies to histone H2B. Antibodies to the other histones (H2A, H3, and H4) were less apparent. Five of the 11 patients (and two others in the remainder of the sample of 24) also had antibodies to a small number of nonhistone proteins that are probably components of ribonucleoprotein particles, but there was no obvious correlation between the presence of antihistone antibodies and the known antiribonucleoprotein activity of these sera. Separate determinants on H1 and H2B were demonstrated by immunoblotting with affinity-purified anti-H1 and anti-H2B antibodies derived from serum that showed both specificities. The localization of the determinants within the histone polypeptide chains was shown by immunoblotting with large fragments produced by specific proteolytic or chemical cleavage of the histones. The strongest determinant on H1 was located within the COOH-terminal half, with a weaker determinant being present within the NH2-terminal half; the H2B determinant(s) was located entirely within the NH2-terminal half of the molecule. The selectivity with which the antihistone antibodies in systemic lupus erythematosus are produced against the more exposed histones in the nucleosome (and perhaps against the most exposed regions of these histones) is consistent with the involvement of intact chromatin structures as immunogens in this disease.

Trypsin digestion of core chromatin

Chicken erythrocyte core chromatin was digested with trypsin for 18 h. Five major limit peptides were produced with mol.wts. from 10 800 to 8000 which arose from the central regions of each core histone. The basic amino- and carboxyl-terminal regions of each core histone were digested to small peptides having an average size of less than six amino acids. The small basic peptides, 25% by weight of the total histone, dissociated from the complex and could be removed by dialysis. The five major limit peptides remained bound to the DNA and contained all the secondary structure originally present in the native histones. Trypsin digestion decreased the supercoiling of the DNA in the complex and perturbed the tertiary structure of the histones. By contrast, there were no changes in the secondary structure of the large degraded histone fragments. However, when these were dissociated from the DNA, the secondary structure, which is predominantly alpha-helix, decreased by 50%. It is concluded that DNA binds strongly to the central regions of the core histones via alpha-helical segments on the polypeptide chains.