A helix-turn motif in the C-terminal domain of histone H1

The structural study of peptides belonging to the terminal domains of histo ne H1 can be considered as a step toward the understanding of the function of H1 in chromatin. The conformational properties of the peptide Ac-EPKRSVA FKKT KKEVKKVATPKK (CH-1), which belongs to the C-terminal domain of histone H1 degrees (residues 99-121) and is adjacent to the central globular domai n of the protein, were examined by means of H-1-NMR and circular dichroism. In aqueous solution, CH-1 behaved as a mainly unstructured peptide, althou gh turn-like conformations in rapid equilibrium with the unfolded state cou ld be present. Addition of trifluoroethanol resulted in a substantial incre ase of the helical content. The helical limits, as indicated by (i,i + 3) n uclear Overhauser effect (NOE) cross correlations and significant up-field conformational shifts of the C-alpha protons, span from Pro100 to Val116, w ith Glu99 and Ala117 as N- and C-caps. A structure calculation performed on the basis of distance constraints derived from NOE cross peaks in 90% trif luoroethanol confirmed the helical structure of this region. The helical re gion has a marked amphipathic character, due to the location of all positiv ely charged residues on one face of the helix and all the hydrophobic resid ues on the opposite face. The peptide has a TPKK motif at the C-terminus, f ollowing the alpha-helical region. The observed NOE connectivities suggest that the TPKK sequence adopts a type (I) beta-turn conformation, a sigma-tu rn conformation or a combination of both, in fast equilibrium with unfolded states. Sequences of the kind (S/T)P(K/R)(K/R) have been proposed as DNA b inding motifs. The CH-I peptide, thus, combines a positively charged amphip athic helix and a turn as potential DNA-binding motifs.