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Background Protamines are small basic proteins that condense the DNA in mature spermatozoa. Typical protamines are of simple composition and very arginine-rich, usually in the range of 60-80%. Arginine residues are distributed in a number of stretches separated by neutral amino acids. We have used Fourier transform infrared spectroscopy (FTIR) to gain access for the first time to the secondary structure of protamines in sperm nuclei. This technique is particularly well suited to the study of DNA-bound protamine in whole nuclei since it is not affected by turbidity. Results We show that DNA -bound salmon (salmine) and squid protamines contain α-helix, β-turns and a proportion of other structures not stabilized by intramolecular hydrogen bonding. No β-sheet was observed. In salmine, the α-helix amounted to ~20%, while in squid protamine it reached ~40%. In contrast, the structure not stabilized by intermolecular hydrogen bonding was more abundant in salmine (~40%) than in squid protamine (~20%). Both protamines contained ~40% β-turns. The different helical potential of salmine and squid protamine was confirmed by structure predictions and CD in the presence of trifluoroethanol. Conclusion DNA-bound protamine in sperm nuclei contains large amounts of defined secondary structure stabilized by intramolecular hydrogen bonding. Both salmine and squid protamine contain similar amounts of β-turns, but differ in the proportions of α-helix and non-hydrogen bonded conformations. In spite of the large differences in the proportions of secondary structure motifs between salmon and squid protamines, they appear to be equally efficient in promoting tight hexagonal packing of the DNA molecules in sperm nuclei.

Alzheimer's disease (AD) is one of the most prevalent neurodegenerative diseases of the central nervous system. The aggregation of the amyloid-β peptide, Aβ(1-42), is believed to play an important role in the pathogenesis of AD. Histone H1 is found in the cytoplasm of neurons in AD, and it has been shown to interact with aggregated amyloid-β peptides and with amyloid fibrils. We have used Thioflavin T (ThT) fluorescence enhancement, circular dichroism spectroscopy (CD), coprecipitation, and transmission electron microscopy (TEM) to study the interaction of histone H1 with Aβ(1-42). Both freshly prepared (monomeric) Aβ(1-42) and histone H1 solutions showed negative CD bands typical of the random coil. Mixing Aβ(1-42) and histone H1 led to the loss of the random coil, which was replaced mostly by β-structure. Therefore, both Aβ(1-42) and histone H1 behave as intrinsically disordered proteins with coupled binding and folding. Mutual structure induction demonstrates the interaction of Aβ(1-42) and histone H1. The interaction was confirmed by coprecipitation followed by SDS-PAGE. Mutual structure induction was also observed with the H1 terminal domains. Incubation of Aβ(1-42) for 1 week in the presence of histone H1 led to the formation of laminar aggregates and thick bundles, characterized by the parallel association of large numbers of fibrils. The aggregates were particularly large and ordered with the H1 subtype H1.2. Further aging of the complexes led to tight compaction of fibril bundles and to fiber growth. Stabilization of fibril-fibril interactions appeared to be determined by the C-terminal domain of histone H1. In summary, these observations indicate that histone H1 has at least two effects: it helps the folding of Aβ monomers and stabilizes the parallel association of fibrils.

Linker histones are involved in chromatin higher-order structure and gene regulation. We have successfully achieved partial phosphorylation of linker histones in chicken erythrocyte soluble chromatin with CDK2, as indicated by HPCE, MALDI-TOF and Tandem MS. We have studied the effects of linker histone partial phosphorylation on secondary structure and chromatin condensation. Infrared spectroscopy analysis showed a gradual increase of β-structure in the phosphorylated samples, concomitant to a decrease in α-helix/turns, with increasing linker histone phosphorylation. This conformational change could act as the first step in the phosphorylation-induced effects on chromatin condensation. A decrease of the sedimentation rate through sucrose gradients of the phosphorylated samples was observed, indicating a global relaxation of the 30-nm fiber following linker histone phosphorylation. Analysis of specific genes, combining nuclease digestion and qPCR, showed that phosphorylated samples were more accessible than unphosphorylated samples, suggesting local chromatin relaxation. Chromatin aggregation was induced by MgCl2 and analyzed by dynamic light scattering (DLS). Phosphorylated chromatin had lower percentages in volume of aggregated molecules and the aggregates had smaller hydrodynamic diameter than unphosphorylated chromatin, indicating that linker histone phosphorylation impaired chromatin aggregation. These findings provide new insights into the effects of linker histone phosphorylation in chromatin condensation.

Chicken erythrocyte nuclei were digested with micrococcal nuclease and fractionated by centrifugation in low-salt buffer into soluble and insoluble fractions. Post-translational modifications of the purified linker histones of both fractions were analyzed by LC–ESI–MS/MS. All six histone H1 subtypes (H1.01, H1.02, H1.03, H1.10, H1.1L and H1.1R) and histone H5 were identified. Mass spectrometry analysis enabled the identification of a wide range of PTMs, including Nα-terminal acetylation, acetylation, formylation, phosphorylation and oxidation. A total of nine new modifications in chicken linker histones were mapped, most of them located in the N-terminal and globular domains. Relative quantification of the modified peptides showed that linker histone PTMs were differentially distributed among both chromatin fractions, suggesting their relevance in the regulation of chromatin structure. The analysis of our results combined with previously reported data for chicken and some mammalian species showed that most of the modified positions were conserved throughout evolution, highlighting their importance in specific linker histone functions and epigenetics. Biological significance Post-translational modifications of linker histones could have a role in the regulation of gene expression through the modulation of chromatin higher-order structure and chromatin remodeling. Finding new PTMs in linker histones is the first step to elucidate their role in the histone code. In this manuscript we report nine new post-translational modifications of the linker histones from chicken erythrocytes, one in H5 and eight in the H1 subtypes. Chromatin fractionated by centrifugation in low-salt buffer resulted in two fractions with different contents and compositions of linker histones and enriched in specific core histone PTMs. Of particular interest is the fact that linker histone PTMs were differentially distributed in both chromatin fractions, suggesting specific functions. Future studies are needed to establish the interplay between PTMs of linker and core histones in order to fully understand chromatin regulation. A protein sequence alignment summarizing the PTMs found to date in chicken, mouse, rat and humans showed that, while many of the modified positions were conserved between these species, the type of modification often varied depending on the species or the cellular type. This finding suggests an important role for the PTMs in the regulation of linker histone functions.

Histone H1 is involved in chromatin structure and gene regulation. H1 also performs functions outside cell nuclei, which may depend on its properties as a lipid-binding protein. The H1 CTD behaves as an intrinsically disordered protein (IDP) with coupled binding and folding. Here, we used neutral detergents and anionic SDS to study the contribution of hydrophobic interactions to the folding of the CTD. In the presence of neutral detergents, the CTD folded with proportions of secondary structure motifs similar to those observed in the DNA complexes. These results identify a folding pathway for the CTD based on hydrophobic interactions, and independent of charge compensation. The CTD is phosphorylated to different extents by cyclin-dependent kinases. The general effect of phosphorylation in the presence of detergents was a decrease in the α-helix content and an increase in that of the β-structure. The greatest effect was observed in the fully phosphorylated CTD (three phosphate groups) in the presence of anionic SDS (7:1, detergent/CTD molar ratio); in these conditions, the CTD became an all-β protein, with 83% β-structure and no α-helix. The CTD in all-β conformation readily formed ribbon-like fibers. The entire H1 also formed fibers when fully phosphorylated in the CTD. Fibers were of the amyloid type, as judged by strong birefringence in the presence of Congo red and thioflavin fluorescence enhancement. Amyloid fiber formation was only observed in SDS, suggesting that it requires the joint effects of partial charge neutralization and hydrophobic interactions, together with the all-β potential provided by full phosphorylation.

H1 linker histones are involved both in the maintenance of higher-order chromatin structure and in gene regulation. Histone H1 exists in multiple isoforms, is evolutionarily variable and undergoes a large variety of post-translational modifications. We review recent progress in the understanding of the folding and structure of histone H1 domains with an emphasis on the interactions with DNA. The importance of intrinsic disorder and hydrophobic interactions in the folding and function of the carboxy-terminal domain (CTD) is discussed. The induction of a molten globule-state in the CTD by macromolecular crowding is also considered. The effects of phosphorylation by cyclin-dependent kinases on the structure of the CTD, as well as on chromatin condensation and oligomerization, are described. We also address the extranuclear functions of histone H1, including the interaction with the β-amyloid peptide.

The percent identity matrices of two sequence multiple alignments between linker histones from chicken and mammalian species are described. Linker histone protein sequences for chicken, mouse, rat and humans, available on public databases were used. This information is related to the research article entitled “Identification of novel post-translational modifications in linker histones from chicken erythrocytes”published in the Journal of Proteomics [1].

Linker histone H1 plays an important role in chromatin folding. Phosphorylation by cyclin-dependent kinases is the main post-translational modification of histone H1. We studied the effects of phosphorylation on the secondary structure of the DNA-bound H1 carboxy-terminal domain (CTD), which contains most of the phosphorylation sites of the molecule. The effects of phosphorylation on the secondary structure of the DNA-bound CTD were site-specific and depended on the number of phosphate groups. Full phosphorylation significantly increased the proportion of beta-structure and decreased that of alpha-helix. Partial phosphorylation increased the amount of undefined structure and decreased that of alpha-helix without a significant increase in beta-structure. Phosphorylation had a moderate effect on the affinity of the CTD for the DNA, which was proportional to the number of phosphate groups. Partial phosphorylation drastically reduced the aggregation of DNA fragments by the CTD, but full phosphorylation restored to a large extent the aggregation capacity of the unphosphorylated domain. These results support the involvement of H1 hyperphosphorylation in metaphase chromatin condensation and of H1 partial phosphorylation in interphase chromatin relaxation. More generally, our results suggest that the effects of phosphorylation are mediated by specific structural changes and are not simply a consequence of the net charge.