Abstract
Previous studies showed that the nuclear phosphoprotein nucleoplasmin performs the first stage of chromatin decondensation of Xenopus sperm at fertilization. It binds and removes sperm basic proteins replacing them with histones. We now show that this activity depends upon the massive hyperphosphorylation of nucleoplasmin that occurs when oocytes mature into eggs. Egg extracts or purified hyperphosphorylated egg nucleoplasmin decondense sperm chromatin and remove sperm basic proteins much faster than oocyte extracts or hypophosphorylated oocyte nucleoplasmin. Furthermore, dephosphorylation of egg nucleoplasmin slows sperm decondensation and prevents basic protein removal from sperm chromatin. We conclude that hyperphosphorylation of nucleoplasmin is used to modulate the rapid changes in chromatin structure that accompany early development in Xenopus.
Highlights
Oocyte cytoplasm does not decondense injected nuclei at all [3]
Egg Extracts Decondense Sperm Nuclei More Efficiently than Oocyte Extracts—We have tested the possibility that nucleoplasmin phosphorylation is required for efficient sperm decondensation at fertilization
We have shown that different abilities of eggs and oocytes to decondense chromatin rapidly are mimicked by extracts and that this difference can be attributed to the state of the nucleoplasmin they each contain (Figs. 2 and 3)
Summary
Oocyte cytoplasm does not decondense injected nuclei at all [3]. We have investigated the molecular basis of these differences. We have compared the efficiency of sperm chromatin decondensation in oocyte versus egg extracts and in purified nucleoplasmin from oocytes and eggs. We observe that the remodeling of sperm chromatin, associated with decondensation, occurs in egg extracts and egg nucleoplasmin but not oocyte extracts or oocyte nucleoplasmin.
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