Localization of mitotic‐apparatus‐associated 51‐kD protein in unfertilized and fertilized sea urchin eggs

The 51‐kD protein, a protein component of the mitotic apparatus in sea urchin eggs, is involved in the aster‐forming activity previously shown in vitro [Toriyama et al., 1988]. Postembedding immunofluorescent labelings of eggs from fertilization through first cleavage showed that the 51‐kD protein is localized in sperm asters, centrosomal regions, spindles, basal regions of astral microtubules, and regions surrounding daughter nuclei at telophase in situ. Immunofluorescence and immunoblot analyses detected the 51‐kD protein uniformly in unfertilized eggs, but not in spermatozoa. When unfertilized eggs were treated with taxol, the 51‐kD protein was shown to be associated with taxol‐induced cytasters. Immunoblot analysis revealed that similar protein species are present in the mitotic apparatus of other species of sea urchin. It was suggested that the 51‐kD protein may be involved in microtubule nucleation and microtubule matrix in sea urchin eggs in vivo.