Cytoplasm of sea urchin unfertilized eggs contains a nucleosome remodelingactivity

After fertilization the sea urchin sperm nucleus transforms into the male p ronucleus which later fuses with the female pronucleus re-establishing the diploid genome of the embryo. This process requires remodeling of the sperm chromatin structure including the replacement of the sperm histones by mat ernally derived cleavage stage histone variants. In recent years, a group o f protein complexes that promote chromatin-remodeling in an ATP-dependent m anner have been described. To gain understanding into the molecular mechani sms operating during sea urchin male pronuclei formation, we analyzed wheth er chromatin-remodeling activity was present in unfertilized eggs as well a s during early embryogenesis. We report that in the sea urchin Tetrapygus n iger, protein extracts from the cytoplasm but not from the nucleus, of unfe rtilized eggs exhibit ATP-dependent nucleosome remodeling activity. This cy tosolic activity was not found at early stages of sea urchin embryogenesis. In addition, by using polyclonal antibodies in Western blot analyses, we f ound that an ISWI-related protein is primarily localized in the cytoplasm o f the sea urchin eggs. Interestingly, SW12/SNF2-related proteins were not d etected neither in the nucleus nor in the cytoplasm of unfertilized eggs. D uring embryogenesis, as transcriptional activity is increased an ISWI-relat ed protein is found principally in the nuclear fraction. Together, our resu lts indicate that the cytoplasm in sea urchin eggs contains an ATP-dependen t chromatin-remodeling activity, which may include ISWI as a catalytic subu nit. (C) 2001 Wiley-Liss, Inc.