A stable alpha-helical element in the carboxy-terminal domain of free and chromatin-bound histone H1 from sea urchin sperm.

Abstract

The carboxy‐terminal domain (residues 121‐248) of sea urchin sperm‐specific H1 is not random coil but partly alpha‐helical, even in 1 mM sodium phosphate, pH 7. The helix resides in a 57 residue proline‐free segment which, in the intact histone, immediately abuts the central globular domain. The proline‐free region, which is rich in lysine and alanine, is relatively resistant to tryptic digestion when the carboxy‐terminal domain is bound to DNA. Two (overlapping) resistant peptides are shown by circular dichroism measurements to be substantially alpha‐helical in 1 mM sodium phosphate and to increase in helix content to approximately 70% in 1 M NaCLO4. Tryptic digestion of chromatin gives resistant fragments containing both the globular domain and the contiguous proline‐free segment, strongly suggesting that the alpha‐helical segment also exists in chromatin, where it would be ideally placed to direct the path of the linker DNA entering or leaving the nucleosome. The linker in sea urchin sperm chromatin is long (approximately 74 bp), and the unusually long alpha‐helical segment in the carboxy‐terminal tail of sperm H1 which has amphipathic character due to the alanine distribution, and is likely to be curved, may be a special feature tailored to organize it.

Helical structure of basic proteins from spermatozoa

“…The C-terminal half of histone H1 from sea urchin sperm had been previously studied by Clark et al [4] and later by Hill et al [15], who showed by proteolytic digestion that the main helical region is conserved upon interaction with DNA. Our analysis is in full agreement with these previous studies and, in fact, the H1 protein is used in the present paper as a reference protein.…”

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“…Possible mechanisms by which TFE affects polypeptide structure include enhancement of internal hydrogen bonding, the disruption of water structure and preferential solvation of certain groups of the polypeptide chain. In previous studies with entire H1 molecules and model peptides, repulsion between positively charged residues was assumed to counteract the tendency toward the a-helical structurẽ Walters & Kaiser, 1985;Clark et al, 1988;Hill et al, 1989;Johnson et al, 1994!. Organic solvents as TFE decrease the dielectric constant of the medium.…”

“…This kind of arrangement could contribute to the clustering of the positive charges over a short region of the DNA and, since the peptide is adjacent to the globular domain, it could affect the conformation of the initial part of the linker DNA at the exit of the nucleosome. This could have important consequences in those cases where gene regulation is mediated by the tail domains~Lee & Archer, 1998; Dou et al, 1999!. It has been proposed that the C-terminal domain of H1 may contain a-helical elements~Clark et Hill et al, 1989! andb-turns~Erard et al, 1990!…”

“…H1 terminal domains have little or no structure in solution. The C-terminal domain, however, might acquire a substantial proportion of a-helical structure upon interaction with DNA~Clark et al, 1988;Hill et al, 1989!. The C-terminal domain is rich in lysine, alanine, and proline and binds to the linker DNA.…”

“…The low degree of defined secondary structure in the C-terminal domain in aqueous solution may be due to the electrostatic repulsion between the positively charged lysine side chains. Trifluoroethanol and other agents known to stabilize secondary structure induce variable amounts of helical structure in the C-terminal domain of H1 subtypes~Clark et al, 1988;Hill et al, 1989!. Here, we study the conformational properties of a peptide belonging to the C-terminal domain of the H1 subtype H18 by highresolution NMR and CD. The peptide is adjacent to the globular domain and contains the longest proline-free fragment in the C-terminal domain of H18.…”