ABSTRACT
The sea urchin egg receptor for sperm is a 350 kDa glycoprotein containing a large extracellular domain that contains the sperm binding site, a transmembrane domain and a short COOH- terminal intracellular domain. During oogenesis, the receptor protein is first detected in Golgi-associated vesicles and cortical granules. Not until the egg is mature does the receptor appear on the cell surface; at this stage the intact receptor is found in approximately equal quantities on the egg cell surface and in cortical granules. As a potentially unique type of receptor, we were interested in its fate following fertilization. Several techniques have revealed that, following sperm binding, the amount of receptor markedly decreases. Using western blot analysis as well as direct measurement of the receptor protein, it was found that the membrane-bound form of the receptor rapidly disappeared following sperm binding to the egg, with only 3% of the receptor remaining after 30 s. Analysis by immupoelectron microscopy revealed that 30 s after sperm binding, 30% of the initial level of receptor was present. This remaining 30% was found mostly within the perivitelline space formed by the raised fertilization envelope. The disparity between these two sets of results (i.e. 3 vs 30%) is most likely accounted for by the exocytosis of receptor molecules from cortical granules; this fraction of the receptor would have been lost during isolation of the membrane-bound form of the receptor. Thus, unlike other cell surface receptors, the sea urchin egg receptor for sperm is not endocytosed and recycled following ligand binding. Rather, it disappears, presumably as a result of proteolysis. Transiently, the cortical granule form of the receptor is found released into the perivitelline space where it may bind to sperm and thereby prevent polyspermy. Despite the apparent secretion of this form of the receptor, experiments with antibodies to the extracellular and intracellular domains indicate that the receptors in cortical granules and in the plasmic membrane are similar, if not identical.
ABSTRACT
When proteins isolated from spicules of Strongylocentrotus purpuratus embryos were examined by western blot analysis, a major protein of approximately 43 kDa was observed to react with the monoclonal antibody, mAb 1223. Previous studies have established that this antibody recognizes an asparagine-linked, anionic carbohydrate epitope on the cell surface glycoprotein, msp130. This protein has been shown to be specifically associated with the primary mesenchyme cells involved in assembly of the spicule. Moreover, several lines of evidence have implicated the carbohydrate epitope in Ca2+ deposition into the growing spicule. The 43 kDa, spicule matrix protein detected with mAb 1223 also reacted with a polyclonal antibody to a known spicule matrix protein, SM30. Further characterization experiments, including deglycosylation using PNGaseF, two-dimensional electrophoresis, and immunoprecipitation, verified that the 43 kDa spicule matrix protein had a pl of approximately 4.0, contained the carbohydrate epitope recognized by monoclonal antibody mAb 1223 and reacted with anti-SM30. Electron microscopy confirmed the presence of proteins within the demineralized spicule that reacted with mAb 1223 and anti-SM30. We conclude that the spicule matrix protein, SM30, is a glycoprotein containing carbohydrate chains similar or identical to those on the primary mesenchyme cell membrane glycoprotein, msp130.
