Characterization of the nucleic acid-binding activities of the isolated amino-terminal head domain of the intermediate filament protein vimentin reveals its close relationship to the DNA-binding regions of some prokaryotic single-stranded DNA-binding proteins.

In order to demonstrate that the nucleic acid-binding activities of vimentin are dictated by its Arg-rich N-terminal head domain, this was cut off at position Lys96 with lysine-specific endoproteinase and analysed for its capacity to associate with a variety of synthetic and naturally occurring nucleic acids. The isolated polypeptide (vim NT) showed a preference for single-stranded (ss) polynucleotides, particularly for ssDNAs of high G-content. A comparison of the sequence and predicted secondary structure of vim NT with that of two prokaryotic ssDNA-binding proteins, G5P and G32P of bacteriophages fd and T4, respectively, revealed that the nucleic acid-binding region of all three polypeptides is almost entirely in the beta-conformation and characterized by a very similar distribution of aromatic amino acid residues. A partial sequence of vim NT can be folded into the same beta-loop structure as the DNA-binding wing of G5P of bacteriophage fd and related viruses. As in the case of G5P, nitration of the Tyr residues with tetranitromethane was blocked by single-stranded nucleic acids. This and spectroscopic data indicate intercalation of the Tyr aromatic ring systems between the bases of the nucleic acids and thus the contribution of a stacking component to the binding reaction. The binding was accompanied by significant changes in the ultraviolet absorption spectra of both vim NT and single-stranded nucleic acids. Upon mixing of vim NT with nucleic acids, massive precipitation of the reactants occurred, followed by the quick rearrangement of the aggregates with the formation of specific and soluble association products. Even at very high ionic strengths, at which no electrostatic reaction should be expected, a distinct fraction of vim NT incorporated naturally occurring ssRNAs and ssDNAs into fast sedimenting complexes, suggesting co-operative interaction of the polypeptide with the nucleic acids. In electron microscopy, the complexes obtained from 28 S rRNA appeared as networks of extended nucleic acid strands densely covered with vim NT, in contrast to the compact random coils of uncomplexed RNA. The networks produced from fd DNA were heterogeneous in appearance and their nucleoprotein strands in rare cases were very similar to the rod-like structures of G5P-fd DNA complexes.