An inducible helix–Gly–Gly–helix motif in the N-terminal domain of histone H1e: A CD and NMR study

Knowledge of the structural properties of linker histones is important to the understanding of their role in higher-order chromatin structure and gene regulation. Here we study the conformational properties of the peptide Ac-EKTPVKKKARKAAGGAKRKTSG-NH(2) (NE-1) by circular dichroism and (1)H-NMR. This peptide corresponds to the positively charged region of the N-terminal domain, adjacent to the globular domain, of mouse histone H1e (residues 15-36). This is the most abundant H1 subtype in many kinds of mammalian somatic cells. NE-1 is mainly unstructured in aqueous solution, but in the presence of the secondary-structure stabilizer trifluoroethanol (TFE) it acquires an alpha-helical structure. In 90% TFE solution the alpha-helical population is approximately 40%. In these conditions, NE-1 is structured in two alpha-helices that comprise almost all the peptide, namely, from Thr17 to Ala27 and from Gly29 to Thr34. Both helical regions are highly amphipathic, with the basic residues on one face of the helix and the apolar ones on the other. The two helical elements are separated by a Gly-Gly motif. Gly-Gly motifs at equivalent positions are found in many vertebrate H1 subtypes. Structure calculations show that the Gly-Gly motif behaves as a flexible linker between the helical regions. The wide range of relative orientations of the helical axes allowed by the Gly-Gly motif may facilitate the tracking of the phosphate backbone by the helical elements or the simultaneous binding of two nonconsecutive DNA segments in chromatin.