Biochemical studies of the envelope transformations in Xenopus laevis eggs.

The envelopes that enclose the eggs of the amphibian Xenopus laevis were isolated and examined for biochemical correlates of the ultrastructural and sperm penetrability differences among the coelomic egg envelope (CE), the vitelline envelope (VE), and the fertilization envelope (FE). By sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), the 43,000 molecular weight glycoproteins of CEs were found to be converted to components with molecular weights of 41,000 in VEs; also, a protein with a molecular weight of 57,000 was added to the envelope during the CE-to-VE conversion. The molecular weights of two components decreased during the VE-to-FE conversion, from 69,000 and 64,000 in the VE to 66,000 and 61,000 in the FE. Components from the cortical granules and the innermost jelly coat were also added to the newly-formed FE. As detected by iodination with lactoperoxidase or IODOGEN, both the CE-to-VE and the VE-to-FE conversions caused conformational changes in envelope glycoproteins. Peptide mapping demonstrated that the 43,000 molecular weight components of CE were precursors to the 41,000 molecular weight components of VE and the 69,000 and 64,000 molecular weight components of VE were precursors to the 66,000 and 61,000 molecular weight components of FE. The CE-to-VE conversion presumably occurs in the first portion of the oviduct. Experiments probing the VE-to-FE conversion demonstrated the need for an intact jelly coat for the molecular weight changes to occur. Sperm were not required for the envelope alteration; the SDS-PAGE pattern of envelopes from jellied eggs activated with the Ca++-ionophore A23187 were indistinguishable from the FE. These studies show that there are molecular correlates of the morphological and biological differences among the envelopes. The CE-to-VE and the VE-to-FE conversions follow a similar pattern: in both cases, material is added to the envelope and there are changes in the molecular weights of some of the components.