Post-translational modifications of Trypanosoma cruzi histone H4

Histone H1 of Trypanosoma cruzi is concentrated in the nucleolus region and disperses upon phosphorylation during progression to mitosis

Phosphorylation of histone H1 is intimately related to the cell cycle progression in higher eukaryotes, reaching maximum levels during mitosis. We have previously shown that in the flagellated protozoan Trypanosoma cruzi, which does not condense chromatin during mitosis, histone H1 is phosphorylated at a single cyclin-dependent kinase site. By using an antibody that recognizes specifically the phosphorylated T. cruzi histone H1 site, we have now confirmed that T. cruzi histone H1 is also phosphorylated in a cell cycle-dependent manner. Differently from core histones, the bulk of nonphosphorylated histone H1 in G(1) and S phases of the cell cycle is concentrated in the central regions of the nucleus, which contains the nucleolus and less densely packed chromatin. When cells pass G(2), histone H1 becomes phosphorylated and starts to diffuse. At the onset of mitosis, histone H1 phosphorylation is maximal and found in the entire nuclear space. As permeabilized parasites preferentially lose phosphorylated histone H1, we conclude that this modification promotes its release from less condensed and nucleolar chromatin after G(2).

Acetylation of histone H4K4 is cell cycle regulated and mediated by HAT3 in Trypanosoma brucei

Post-translational histone modifications have been studied intensively in several eukaryotes. It has been proposed that these modifications constitute a 'histone code' that specifies epigenetic information for transcription regulation. With a limited number of histone-modifying enzymes, implying less redundancy, Trypanosoma brucei represents an excellent system in which to investigate the function of individual histone modifications and histone-modifying enzymes. In this study, we characterized the acetylation of lysine 4 of histone H4 (H4K4), the most abundant acetylation site in T. brucei histones. Because of the large sequence divergence of T. brucei histones, we generated highly specific antibodies to acetylated and unmodified H4K4. Immunofluorescence microscopy and Western blots with sorted cells revealed a strong enrichment of unmodified H4K4 in S phase and suggested a G1/G0-specific masking of the site, owing to non-covalently binding factors. Finally, we showed that histone acetyltransferase 3 (HAT3) is responsible for H4K4 acetylation and that treatment of cells with the protein synthesis inhibitor cycloheximide led to an almost instantaneous loss of unmodified H4K4 sites. As HAT3 is located inside the nucleus, our findings suggest that newly synthesized histone H4 with an unmodified K4 is imported rapidly into the nucleus, where it is acetylated, possibly irreversibly.

Shared epigenetic mechanisms control virulence factors in protozoan parasites

Protozoan pathogens have evolved countermeasures to avoid immune clearance and prolong the period of infection in their vertebrate hosts. The type and degree of immune escape strategies depends on the in vivo 'lifestyle' the pathogen has adopted. Here we describe how parasites use different strategies to coordinate their expression of phenotypic variation, which is used in many cases to fool the immune system, or to successfully invade new host cells. Recent insights using modern molecular biology techniques show that this is achieved via a coordinated manner of action of different epigenetic factors such as histone marks, subnuclear localization, or novel unknown mechanism(s). This emerging field may have an enormous impact on disease therapy.

Histone modifications in Trypanosoma brucei

Several biological processes in Trypanosoma brucei are affected by chromatin structure, including gene expression, cell cycle regulation, and life-cycle stage differentiation. In Saccharomyces cerevisiae and other organisms, chromatin structure is dependent upon posttranslational modifications of histones, which have been mapped in detail. The tails of the four core histones of T. brucei are highly diverged from those of mammals and yeasts, so sites of potential modification cannot be reliably inferred, and no cross-species antibodies are available to map the modifications. We therefore undertook an extensive survey to identify posttranslational modifications by Edman degradation and mass spectrometry. Edman analysis showed that the N-terminal alanine of H2A, H2B, and H4 could be monomethylated. We found that the histone H4 N-terminus is heavily modified, while, in contrast to other organisms, the histone H2A and H2B N-termini have relatively few modifications. Histone H3 appears to have a number of modifications at the N-terminus, but we were unable to assign many of these to a specific amino acid. Therefore, we focused our efforts on uncovering modification states of H4. We discuss the potential relevance of these modifications.

Introducing histone modification in trypanosomes

Nuclear DNA is wrapped around histones. Vigorous research over the past decade has established a central role for histone post-translational modification in controlling the DNA-protein interactions that are required for successful growth and propagation. Recent work now provides a description of acetylated and methylated residues in the divergent trypanosome core histones. Future studies should provide insights into the genomic distribution of each modification and their roles in growth and pathogenesis.