Functional role of a high mol mass protein complex in the sea urchin yolk granule

We have investigated the biochemical and functional characteristics of the major protein constituents of the yolk granule organelle present in sea urchin eggs and embryos. Compositional analysis, using sodium dodecyl sulfate polyacrylamide gel electrophoresis, revealed distinctly different polypeptide patterns under reducing and non-reducing conditions. In the presence of reducing agent, a 240 kDa species dissociated into polypeptides of apparent mol mass 160, 120 and 90 k. The relatedness of these polypeptides to the 240 kDa species was demonstrated in protein gel blot and peptide mapping analyses. The profile of yolk granule polypeptides was dynamic during embryonic development with the disappearance of the 160 kDa species and the coincidental appearance of lower mol mass polypeptides. However, the 240 kDa complex was detected even after the disappearance of the 160 kDa polypeptide. The 240 kDa complex was released from yolk granules in the absence of calcium and the purified species was shown to bind liposomes in a calcium-dependent manner. In addition, the 240 kDa complex possessed a calcium-dependent, liposome aggregating activity. The 240 kDa species could also induce the aggregation of yolk granules, previously denuded of the complex following treatment with either ethylenediaminetetraacetic acid or trypsin. Collectively, these results demonstrate the dynamic characteristics of the yolk granule 240 kDa protein complex and offer insights into a possible functional role.