Analysis of changes in a yolk glycoprotein complex in the developing sea urchin embryo

A sea urchin yolk glycoprotein complex (YGC) was isolated from several developmental stages by velocity centrifugation on sucrose gradients. The YGCs were analyzed by SDS-polyacrylamide gel electrophoresis to determine if the molecular composition of the YGC was changing during development. The mass of the YGC did not change with development. However, as development proceeded there were significant changes in the glycoprotein composition of the YGC isolated from either Lytechinus pictus or Strongylocentrotus purpuratus embryos. In both species the YGC isolated from eggs contained three major glycoproteins. The most abundant one had an apparent molecular weight of 190,000 and was designated GP-190. During development the three major egg YGC glycoproteins decreased in relative amounts as intermediate-molecular-weight glycoproteins increased. While these changes were detected in YGCs isolated from either species, the rate of change was much greater in S. purpuratus than in L. pictus. The most significant difference was observed in the rate of decrease in GP-190. In S. purpuratus, GP-190 showed a significant decrease by 8-10 hr postfertilization, while a similar decrease did not occur in L. pictus YGCs until 72 hr postfertilization. To determine how these changes were occurring, both amino acid and carbohydrate analyses were done on the YGC isolated from various stages. From examination of these data, it appeared that the molecular composition of the YGC was changing via very limited proteolysis. The intermediate and low-molecular-weight glycoproteins generated apparently remain assembled in the YGC, thus conserving its mass.