Poly(ADP-ribosylation) protects maternally derived histones from proteolysis after fertilization

Abstract

Fertilization in sea urchins is followed by the replacement of sperm-specific histones by cleavage-stage histone variants recruited from maternal stores. Such remodelling of zygote chromatin involves a cysteine proteinase that degrades the sperm-specific histones in a selective manner, leaving the maternal cleavage-stage histone variants intact. The mechanism that determines the selectivity of the sperm-histone-selective proteinase (SpH-proteinase) was analysed by focusing on the post-translational modification status of both sets of histones. It has previously been reported that only native cleavage-stage histones are poly(ADP-ribosylated), whereas the sperm-specific histones are not modified. To determine whether the poly(ADP-ribose) moiety afforded protection from degradation, the ADP-ribose polymers were removed from the cleavage-stage histones in vitro; these proteins were then assayed as potential substrates of the SpH-proteinase. Strikingly, the cleavage-stage histone variants were extensively degraded after the enzymic removal of their ADP-ribose moieties. In addition, the SpH cysteine proteinase was not inhibited by isolated poly(ADP-ribose) polymers. Consequently, only poly(ADP-ribosylated) cleavage-stage histone variants are protected from proteolysis. These results demonstrate a novel role for this type of post-translational modification, namely the protection of nuclear proteins against nuclear proteinases after fertilization.

Remodeling of sperm chromatin after fertilization involves nucleosomes formed by sperm histones H2A and H2B and two CS histone variants

“…This protease degrades sperm histones leaving the maternal CS histone variants intact [Imschenetzky et al, 1997]. The protection of CS histone variants against proteolysis is due to their extensive poly(ADP-ribosylation) [Morin et al, 1999a]. Alternatively, we have also demonstrated that sperm specific histones H1 and H2B are protected against this protease by their phosphorylation [Morin et al, 1999b].…”

Chromatin remodeling during sea urchin early development: molecular determinants for pronuclei formation and transcriptional activation

“…Interestingly, it was demonstrated that this substrate selectivity is mainly determined by the poly(ADP-ribose) moiety present in the CS variants, as only the modified proteins were protected from degradation (Morin et al, 1999a). It was also found that phosphorylation of SpH1 and SpH2B histones protect them from degradation (Morin et al, 1999b).…”

During male pronuclei formation chromatin remodeling is uncoupled from nucleus decondensation

“…Furthermore, we have identified a nuclear cysteine-protease that degrades the SpH leaving the maternal CS histones unaffected [Imschenetzky et al, 1997]. The activity of this enzyme is modulated by post-translational modifications of its substrates, phosphorylation protects SpH1 and SpH2B from degradation [Morin et al, 1999a], and poly(ADP-ribosylation) blocks the proteolysis of CS histone variants [Morin et al, 1999b]. It is unknown yet if this protease may catalyze the degradation of the SpH while these histones are organized as nucleosome cores, or if the SpH should be removed before degradation.…”