Sequence and characterization of a sperm-specific histone H1-like protein of Mytilus californianus

The major protein fraction of the protamine-like PL-II* (phi 2B) from the sperm of Mytilus californianus has been sequenced and characterized. Immunological and sequence analyses unequivocally show that this protein is indeed a member of the histone H1 family. Along with proteins of the histone H1 class, the protein also shows cross-reactivity and sequence identity, in its NH2-terminal region, with the major protamine-like protein component of Mytilus sperm: PL-III (phi 1), of smaller molecular mass. Indeed it is the unusual repetitive sequence motif of the NH2-terminal domain of PL-II* that bestows to this protein its protamine-like nature. Fourier-transform infrared spectroscopy spectroscopy indicates that the protein contains considerable secondary structure: 18% alpha-helix, 21% beta-sheet, 39% turns and bends, 22% random coil. At the higher levels of structure, PL-II* exhibits ionic strength-dependent folding which is indistinguishable from that of histone H5, as monitored by fluorescence anisotropy.