Abstract

After fertilization the sea urchin sperm nucleus transforms into the male pronucleus 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 maternally derived cleavage stage histone variants. In recent years, a group of protein complexes that promote chromatin-remodeling in an ATP-dependent manner have been described. To gain understanding into the molecular mechanisms operating during sea urchin male pronuclei formation, we analyzed whether chromatin-remodeling activity was present in unfertilized eggs as well as during early embryogenesis. We report that in the sea urchin Tetrapygus niger, protein extracts from the cytoplasm but not from the nucleus, of unfertilized eggs exhibit ATP-dependent nucleosome remodeling activity. This cytosolic activity was not found at early stages of sea urchin embryogenesis. In addition, by using polyclonal antibodies in Western blot analyses, we found that an ISWI-related protein is primarily localized in the cytoplasm of the sea urchin eggs. Interestingly, SWI2/SNF2-related proteins were not detected neither in the nucleus nor in the cytoplasm of unfertilized eggs. During embryogenesis, as transcriptional activity is increased an ISWI-related protein is found principally in the nuclear fraction. Together, our results indicate that the cytoplasm in sea urchin eggs contains an ATP-dependent chromatin-remodeling activity, which may include ISWI as a catalytic subunit. © 2001 Wiley-Liss, Inc.