Isolation, physicochemical properties, and the macromolecular composition of the vitelline and fertilization envelopes from Xenopus laevis eggs

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Abstract
As a step toward defining in molecular terms the sperm-triggered block to polyspermy reaction established by the egg at fertilization, vitelline (VE) and fertilization (FE) envelopes were isolated from eggs of the South African clawed toad X. laevis and some of their physicochemical properties determined. Envelopes were isolated after lysis of the fertilized or unfertilized eggs by sieving techniques; isolated envelopes retained their in situ morphology as determined by EM. The isolated envelopes had different solubility properties and, in general, VE was more readily dissolved by aqueous solvents than FE, although both could be completely dissolved by detergents or chaotropic agents. Changes in envelope solubility correlated with the progression of the cortical reaction implicating a role for cortical granule material in modifying the solubility properties of the envelope. The VE and FE were composed of protein and carbohydrate with no lipid components detected. As determined by immunodiffusion experiments, the FE contained the same antigens as the VE plus components derived from the cortical granules and the innermost jelly layer, J1. The macromolecular composition of the envelopes was determined by sodium dodecyl sulfate gel electrophoresis. The VE contained at least 11 glycoproteins with MW ranging from 125,000 to < 16,000 with 2 components (40,000 and 33,000) accounting for almost 2/3 of the total stainable material. The FE contained 10 glycoproteins that had the same molecular weights as those in the VE. One glycoprotein component underwent a reduction in MW from 77,000-67,500 when the VE was converted to the FE. This molecular weight change was interpreted as the probable result of limited proteolysis. The FE gel electrophoresis patterns contained macromolecular components derived from the cortical granules and jelly layer, J1, consistent with the immunodiffusion experiments. These components were absent when the FE was prepared in the absence of Ca2+, suggesting a role for Ca2+, in binding the VE, cortical granules, and J1 components together. The conversion of the glycoproteinaceous VE to FE at fertilization is caused by interaction of the VE with components from the cortical granules and jelly layer J1. These interactions are of both a chemical and physical nature.

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