VITELLOGENIN AND LIPOVITELLIN - ZINC PROTEINS OF XENOPUS-LAEVIS OOCYTES

Xenopus laevis vitellogenin is a plasma protein that contains a total of 5 mol of metal/440 kDa dimer, 2 mol of zinc, and 3 mol of calcium ( Montorzi et al. (1994) Biochem. Biophys. Res. Commun. 200, 1407-1413]. There are no other group IIB or transition metals in the molecule. Th e zinc atoms are removed instantaneously by 1,10-phenanthroline (OF) ( pK 4.8). Once internalized by receptor-mediated endocytosis, vitelloge nin is cleaved into multiple polypeptides, i.e., the two lipovitellin subunits (1 and 2) plus phosvitin; these are then stored as microcryst als within yolk platelets. We here show by metal analysis of the indiv idual proteins generated by vitellogenin processing that zinc and calc ium occur in different domains of the vitellogenin polypeptide chain. All of the vitellogenin zinc is present in lipovitellin, in amounts eq ual to 1 mol of zinc/141 kDa. Calcium, in contrast, is detected exclus ively in phosvitin which, in addition, contains 3 mol of magnesium/35 kDa, apparently acquired following vitellogenin entry into the oocyte. The zinc in lipovitellin is removed by OP in a concentration-dependen t manner with a pK of 4.8, identical to that obtained for vitellogenin , and by exposure to acidic conditions (below pH 5). Following removal of zinc, the two lipovitellin subunits remain associated, suggesting that zinc is not involved in their interaction. On exposure to 1% SDS, lipovitellin does dissociate into 106 and 33 kDa subunits. The presen ce of stoichiometric quantities of zinc in both vitellogenin and lipov itellin calls for the study of the hitherto unrecognized biochemistry and functions of these proteins in zinc metabolism and development of the frog oocyte and embryo.