Precursor-Product Relationship between Amphibian Vitellogenin and the Yolk Proteins, Lipovitellin and Phosvitin

Abstract The yolk precursor protein vitellogenin was isolated from plasma of estrogen-treated Xenopus laevis males, and the yolk platelet proteins, lipovitellin and phosvitin, were isolated from oocytes of females previously stimulated with human chorionic gonadoptropin. The molecular weights of the polypeptide chains of vitellogenin and lipovitellin were determined by electrophoresis on sodium dodecyl sulfate polyacrylamide gels and by gel filtration in 6 m guanidine hydrochloride. Both methods yielded a molecular weight of approximately 200,000 for delipidated, S-carboxymethylated vitellogenin. Lipovitellin consisted of two polypeptide chains, with molecular weights of 31,000 and 120,000 and a 1:1 molar ratio, as shown by electrophoresis on sodium dodecyl sulfate polyacrylamide gels. The smaller subunit appeared to be a phosphoprotein with a phosphate content of about 2%. The heavy subunit contained little or no phosphate. Treatment with guanidine hydrochloride did not dissociate lipovitellin into the two subunits: after gel filtration of delipidated, S-carboxymethylated lipovitellin in 6 m guanidine hydrochloride only one component was observed, with a molecular weight of about 140,000. Electrophoresis of 32P-labeled phosvitin on sodium dodecyl sulfate polyacrylamide gels produced two labeled bands with mobilities corresponding to molecular weights of 35,000 and less than 25,000. The total molecular weight of the lipovitellin and phosvitin peptide chains was essentially equal to that of the polypeptide chain of vitellogenin. Vitellogenin incubated with very low concentrations of either trypsin or chymotrypsin dissociated into discrete chains with characteristics very similar to those of the peptide chains present in yolk platelets. The evidence thus supports the view that the vitellogenin precursor is a continuous polypeptide chain which is degraded into specific yolk proteins by proteolytic splitting in regions that are also sensitive to attack by trypsin and chymotrypsin.