Remodeling of sperm chromatin induced in egg extracts of amphibians.

Sperm nuclear basic proteins of Bufo japonicus consist of 2 distinct protamines, whereas those of Xenopus laevis consist of 6 sperm-specific basic proteins (SP1-6) in addition to H3, H4 and smaller amounts of H2A and H2B. Cloning of pertinent cDNAs and partial amino acid sequence studies suggested that these 6 sperm-specific proteins of Xenopus are encoded by 3 distinct genes. Despite differences in their initial compositions of chromatin, sperm nuclei exposed to amphibian egg extracts rapidly decondense, lose sperm-specific basic proteins, and concomitantly form an ordinary nucleosome core consisting of H2A, H2B, H3, H4, and cleavage-stage specific subtype H1X. In this remodeling process, nucleoplasmin plays dual roles as a molecular chaperone, selectively removing sperm-specific basic proteins from, and bringing H2A and H2B to, sperm DNA. Thus remodeling of chromatin is induced even in mammalian (human) sperm nuclei under defined conditions including nucleoplasmin and exogenous histones.