Remodeling of Human Sperm Chromatin Mediated by Nucleoplasmin from Amphibian Eggs

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Abstract
The molecular events associated with decondensation of human sperm nuclei were analyzed by incubating sperm with egg extracts from an amphibian, Bufo japonicus. Acid‐urea‐Triton polyacrylamide gel electrophoresis (AUT‐PAGE) showed that the nuclear basic proteins of human sperm consist mainly of protamines (HPI, HPII) with minor amounts of nucleosomal histones. On incubation of lysolecithin (LC)‐ and dithiothreitol (DTT)‐treated human sperm with the egg extract, the nuclei lost HPI and HPII within 15 min in association with extensive nuclear decondensation, and the acquirement of a whole set of nucleosomal histones. Incubation of LC‐DTT‐sperm with nucleoplasmin purified from Bufo eggs also induced nuclear decondensation and loss of protamines within 30 min. Native‐PAGE and Western blot analyses of incubation medium indicated tight association of the released protamines to nucleoplasmin, strongly suggesting that protamines are removed from sperm nuclei not enzymatically but by their specific binding to nucleoplasmin. On incubation of LC‐DTT‐sperm with nucleoplasmin and exogenous nucleosomal core histones, micrococcal nuclease‐protected DNA fragments were released, although their unit repeat length was slightly less than that of somatic nucleosomes. Thus remodeling of human sperm during fertilization can be mimicked under defined conditions with nucleoplasmin and exogenous histones.