Interaction in vitro of non-epithelial intermediate filament proteins with histones

Non-epithelial intermediate filament (IF) subunit proteins show a high and specific affinity for core histones at physiological ionic strength. When IF proteins are titrated with a mixture of core histones and linker histone H1, in general the latter is totally excluded from complexation and in the adducts formed the moderately-arginine-rich histones H2A and H2B are progressively replaced by the very-arginine-rich histones H3 and H4. At histone saturation, 2 molecules of nonneuronal IF protein bind 1 histone H1 molecule or 8 core histone molecules, whereas due to its glutamic acid-rich, C-terminal extensions one dimer of the 68 kD neurofilament protein associates with 3 molecules of histone H1 or 24 molecules of core histones. The salt stability of the insoluble association products is dependent on the amount and arginine content of the constituent histone species. Removal of the non-alpha-helical N- and C-terminal polypeptides from IF proteins by partial chymotryptic digestion does not affect their histone-binding characteristics. Since core histones are only partially inactivated by limited tryptic digestion, they also appear to react through their alpha-helix-rich central domains; the limit peptide derived from histone H1 is completely inactive at physiological ionic strength. Affinity chromatography of rod domains of IF proteins on core histone-Sepharose 4B and of histones and their limit peptides on vimentin-Sepharose 4B has shown that the interactions involving fractions of histones H3 and H4 are extremely resistant to salt and can be dissociated only with arginine or salt under denaturing conditions. In general, the experimental results revealed close parallels between the association of histones with IF proteins and their interaction with DNA.