The Structure of Sea‐Urchin‐Sperm Histone φ1 (H1) in Chromatin and in Free Solution

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Abstract
The lysine-rich H1-type .vphi.1 from the sperm of the sea urchin A. lixula was subjected to tryptic digestion in free solution at high ionic strength. Two trypsin-resistant peptides G.vphi.1 and LG.vphi.1 were isolated, comprising 81 and 93 amino acids, respectively, i.e., about 1/3 of the intact histone. Circular dichroism and 270-MHz protein NMR were used to demonstrate that the smaller peptide G.vphi.1 contains all the secondary and tertiary structure of intact histone .vphi.1. The resistant peptides represent a compact folded portion of the histone .vphi.1 chain, while the remaining 2/3 is disordered in free solution. Trypsin digestion of A. lixula nuclei, under conditions where histone .vphi.1 remained tightly bound to the chromatin, leads to the same resistant peptide G.vphi.1. In chromatin the chain region corresponding to G.vphi.1 is also folded and inaccessible to trypsin, while the remaining exposed residues are located peripherally and probably in an extended conformation. The rather unusual H1-type histone .phi.1 from sea urchin sperm thus has a 3-domain structure like calf thymus H1 and chicken erythrocyte H5. This 3-domain structure exists in chromatin and is not merely a free-solution artefact.