The coelomic envelope to vitelline envelope conversion in eggs of Xenopus laevis.

An amphibian egg recovered from the body cavity is enclosed by a coelomic egg envelope. Upon transport down the oviduct, the envelope is converted to the vitelline envelope. The coelomic and vitelline envelopes are distinct in terms of sperm penetrability, ultrastructural morphology, and radioiodination profiles. In this study, the macromolecular compositions of these two envelopes were determined. Isolated envelopes were compared by one- and two-dimensional gel electrophoresis, peptide mapping, and radiolabeling. A protein with a molecular weight of 57,000 (57K) was present in the vitelline envelope but was absent in the coelomic envelope. A glycoprotein with a molecular weight of 43K in the coelomic envelope was converted to a component with a molecular weight of 41K in the vitelline envelope. The 43K-molecular weight component of the coelomic envelopes could be radioiodinated by lactoperoxidase but no labeling of the 41K-molecular weight component occurred in the vitelline envelope. Peptide mapping using limited proteolysis established that the 43K-molecular weight component of the coelomic envelope was a precursor to the 41K-molecular weight component of the vitelline envelope. These molecular alterations may underlie the ultrastructural and physiological changes occurring in these envelopes.