Accessibility of tyrosyl residues altered by formation of the histone 2A/2B complex

Histone tyrosine phosphorylation comes of age

Histones were discovered over a century ago and have since been found to be the most extensively posttranslationally modified proteins, although tyrosine phosphorylation of histones had remained elusive until recently. The year 2009 proved to be a landmark year for histone tyrosine (Y) phosphorylation as five research groups independently discovered this modification. Three groups describe phosphorylation of Y142 in the variant histone H2A.X, where it may be involved in the cellular decision making process to either undergo DNA repair or apoptosis in response to DNA damage. Further, one group suggests that phosphorylation of histone H3 on Y99 is crucial for its regulated proteolysis in yeast, while another found that Y41 phosphorylation modulates chromatin architecture and oncogenesis in mammalian cells. These pioneering studies provide the initial conceptual framework for further analyses of the diverse roles of tyrosine phosphorylation on different histones, with far reaching implications for human health and disease.

Extra-nuclear location of histones in activated human peripheral blood lymphocytes and cultured T-cells

Dextrin-2-sulphate (D2S) is a sulphated polysaccharide which inhibits human immunodeficiency virus type 1 infection of T-cells by binding to the cell surface. During our investigations of the nature of this interaction, a cell membrane fraction was prepared by ultracentrifugation from the T-cell line, HPB-ALL. Separation of membrane proteins by sodium dodecyl sulphate (SDS)-polyacrylamide gel electrophoresis and analysis for binding proteins using ligand blotting showed that 3H-D2S bound, in a saturable and displaceable manner, to two regions corresponding to molecular weights of 14,000-18,000 and 28,000-32,000. The N-terminal sequences of two of the major protein components in the 14,000-18,000 region were consistent with those of histones H2B and H3. The presence of histone H2B in the cell membrane preparation was confirmed by immunoblotting and enzyme-linked immunosorbent assay using a specific antibody. Histone standards were used to determine the level of each histone in the cell membrane fraction. In addition, the binding of 3H-D2S to purified histone standards was quantified. These results show that all of the binding of 3H-D2S to proteins in the 14,000-18,000 region of the cell membrane preparation can be attributed to the histones present. In contrast to HPB-ALL cells, a cell membrane fraction from freshly isolated human peripheral blood lymphocytes contained very low levels of histones. However, after culture with phytohaemagglutinin for 3 days the cell membrane fraction contained greatly increased levels of histones. To exclude the possibility of contamination of the cell membrane preparation with histones derived from the nucleus, cell membranes were also prepared using an affinity-based method using polyethyleneimine-cellulose. Immunoblotting of adsorbed plasma membranes showed the presence of histone H2B. SDS-polyacrylamide gels stained for protein also indicated that the preparation contained histones H1, H2A, H3 and H4. In further experiments whole cells were used to avoid contamination from nuclear proteins. Lactoperoxidase mediated 125I labelling, a method specific for radiolabelling cell surface proteins, confirmed the presence of histones H2B, H3 and H4 on the surface of HPB-ALL cells. Also, incubation of HPB-ALL cells or phytohaemagglutinin-activated peripheral blood lymphocytes with D2S caused displacement of histones from the cell surface into the supernatant without altering cell viability. In addition, immunocytochemistry of freshly isolated peripheral blood lymphocytes showed that histone H2B was located predominantly in the nucleus. However, in phytohaemagglutinin-activated peripheral blood lymphocytes immunoreactive material was also prominent in the endoplasmic reticulum and on the plasma membrane.(ABSTRACT TRUNCATED AT 400 WORDS)