Dynamics of Ubiquitin Pools in Developing Sea Urchin Embryos

The sea urchin embryo is a closed metabolic system in which embryogenesis is accompanied by significant protein degradation. We report results which are consistent with a function for the ubiquitinmediated proteolytic pathway in selective protein degradation during embryogenesis in this system. Quantitative solid‐ and solution‐phase immunochemical assays, employing anti‐ubiquitin antibodies, showed that unfertilized eggs of Strongylocentrotus purpuratus have a high content of unconjugated ubiquitin (ca. 8 × 108 molecules), and also contain abundant conjugates involving ubiquitin and maternal proteins. The absolute content of ubiquitin in the conjugated form increases about 13‐fold between fertilization and the pluteus larva stage; 90% or more of embryonic ubiquitin molecules are conjugated to embryonic proteins in hatched blastulae and later‐stage embryos. Qualitatively similar results were obtained with embryos of Lytechinus variegatus. The results of pulse‐labeling and immunoprecipitation experiments indicate that synthesis of ubiquitin in S. purpuratus is developmentally regulated, with an overall increase in synthetic rate of 12‐fold between fertilization and hatching. Regulation is likely to occur at the level of translation, since others have shown that levels of ubiquitin‐encoding mRNA remain virtually constant in echinoid embryos during this developmental interval. The sea urchin embryo should be a useful system for characterizing the role of ubiquitination in embryogenesis.