Intrinsic and extrinsic fluorescence of histones H2A and H2B: a conformational study

Interaction of dipalmitoyl-phosphatidylcholine with calf thymus histone H1

The interaction between dipalmitoyl-phosphatidylcholine and calf thymus histone H1 has been studied. A protein-phospholipid complex, resulting from this interaction, has been isolated by centrifugation in a sucrose gradient. The phospholipid-histone interaction causes an increase in the alpha-helix content of the protein; the corresponding conformational transition is observed by CD studies in the far-u.v. region. The only tyrosine residue of the protein can be advantageously used as an intrinsic fluorescent probe; thus, fluorescence spectra indicate that protein folding induced by phospholipids is concomitant with the tyrosine transfer into a more hydrophobic environment. The trypsin-resistant core of the histone is also folded in the presence of the phospholipid but the conformational transition occurs at lower lipid concentration than for the intact protein. Fluorescence polarization of 1,6-diphenyl-1,3,5-hexatriene indicates that the protein shifts the transition temperature of the phospholipid from 41.5 to 44.0 degrees. Secondary structure prediction of the trypsin-resistant core of the histone indicates the existence of an amphipathic helix that could be responsible for the lipid-protein interaction.

Exposed hydrophobic regions in histone oligomers studied by fluorescence

The interaction of the fluorescent hydrophobic probe 1-anilinonaphthalene-8-sulfonate (ANS) with histone oligomers and with histone H1 has been studied. The enhancement of ANS fluorescence produced by histones Hv (a roughly equimolar mixture of histones H2A, H2B, H3 and H4) in 2.0 M NaCl, pH 7.5, is higher than that produced by histone H1 under identical conditions. In addition, the fact that the wavelength of maximum emission of the H1-ANS complex is larger than that of the Hv-ANS complex indicates that histone H1 has a weaker hydrophobic character than histones Hv. The increase in ANS concentration produces a red shift of the emission maximum of the Hv-ANS complex, indicating some heterogeneity in the ANS binding sites. Both the H2A-H2B and the H3-H4 complexes cause a similar enhancement of the ANS fluorescence. Trypsin digestion of N-terminal sequences of histones Hv produces only small changes in the intensity of ANS fluorescence. This result indicates that the hydrophobic regions of histones Hv to which ANS binds are not located in the N-terminal portions of histone sequences. It is suggested that these exposed hydrophobic regions may be important in the maintenance of chromatin structure.

The influence of primary structure changes on the higher order structure of histone H2B

Structural changes of histones H2B in aqueous media were studied under conditions of varying pH and ionic strength. The techniques involved intrinsic fluorescence of tyrosine residues, extrinsic fluorescence using the hydrophobic probe 8-anilinonaphthalene sulfonate and fluorescence polarization. Secondary structure predictive methods were also used to study potential effects of primary mutations. In spite of many primary structure changes, the predicted effect on a globular structure of H2B is minimal. The results indicate the formation of a major hydrophobic region, the formation of helices and the burial of tyrosine residue 83 (calf), the structural changes of which appear to be identical in all H2B's as ionic strength is increased. The effect of primary structure changes on protein-protein interaction where H2B's are concerned is likely to be negligible, whereas the major effect is likely to be the protein-DNA interaction where H2B could be at least partially responsible for differentiation.