Isolation and Amino Acid Sequence Analysis Reveal an Ancient Evolutionary Origin of the Cleavage Stage (CS) Histones of the Sea Urchin

Abstract

The cleavage stage (CS) H1, H2A, and H2B histones of the sea urchin, which have previously been identified by their distinct electrophoretic mobility on Triton/acid/urea gels, are known to be maternally expressed during oogenesis and have been implicated in chromatin remodeling of the male pronucleus following fertilization. Here, we describe the isolation of these three CS histones by reverse-phase HPLC chromatography. Moreover, a novel CS H3 protein was identified by the same purification procedure. A low incorporation of radioactive amino acids into the CS histones during early development revealed that the bulk of these proteins in the blastula embryo are derived from the maternal pool of the egg. Amino acid analysis, together with the previously described electrophoretic mobilities, unequivocally identified the purified proteins as CS histones. Peptide sequence analysis confirmed the novel nature of the CS variants as they are distantly related to the early, late, and sperm histone subtypes of the sea urchin. The CS H1 protein displays highest sequence similarity with the H1M (B4) histone of Xenopus laevis, indicating that the frog H1M protein may be a vertebrate homologue of the CS H1 histone. These data suggest an ancient evolutionary origin and wide distribution of the CS histone variants.