Phosphorus-31 nuclear magnetic resonance (31P NMR) is used to investigate
acid-labile phosphorylation of
histone H4. 31P NMR detects phosphorylated
histidine residues in in vitro enzymatically phosphorylated H4. The source of
kinase is nuclei from either regenerating rat liver or Walker-256
carcinosarcoma. When regenerating rat liver is the source, 31P NMR spectroscopy on the denatured phosphorylated
protein exhibits a resonance at 5.3 ppm relative to an 85%
orthophosphoric acid external reference. This peak corresponds well with the chemical shift of standard pi-
phosphohistidine scanned under similar conditions.
Sodium dodecyl sulfate (NaDodSO4)--polyacrylamide gel electrophoresis confirms
acid lability. When the source of
kinase is Walker-256
carcinosarcoma, the 31P NMR spectrum contains a resonance at 4.9 ppm which corresponds well with standard tau-
phosphohistidine run under the same conditions. Chemical phosphorylation of H4 has been accomplished by using dipotassium
phosphoramidate which specifically phosphorylated the
imidazole moiety of
histidine at neutral pH. NaDodSO4--polyacrylamide gel electrophoresis confirms
acid lability, and high-pressure liquid chromatography of
protein hydrolysates yields
phosphohistidine. 31P NMR of chemically phosphorylated H4 in a structured state reveals two peaks at 4.8 and 7.3 ppm with line widths of 9 and 55 Hz, respectively. These resonances indicate that both
histidine residues of H4 (His-18 and His-75) are phosphorylated, the latter relatively immobile and the former relatively free in
solution. 31P NMR studies on chemically phosphorylated
peptide fragments of H4, namely, H4(1-23) and H4(38-102), confirm this model of H4 structure.