All vertebrate eggs have extracellular matrices, referred to as the zona pe
llucida in Mus musculus and the vitelline envelope in Xenopus laevis. The m
ouse zona, composed of three sulfated glycoproteins (ZP1, ZP2, ZP3), is cri
tical for fertilization and early development, and mice lacking a zona pell
ucida produce no live offspring. The primary structures of mouse ZP1 (623 a
mino acids), ZP2 (713 amino acids) and ZP3 (424 amino acids) have been dedu
ced from full-length cDNAs, but posttranslational modifications result in m
ature zona proteins with molecular masses of 200-180 kDa, 140-120 kDa, and
83 kDa, respectively. The vitelline envelope forms a similar structure arou
nd Xenopus eggs and contains three glycoproteins that are structurally rela
ted (39-48% amino acid similarity) to the three mouse zona proteins. To inv
estigate whether the structural semblances are sufficient to allow incorpor
ation of the mouse zona proteins into the Xenopus vitelline envelope, cappe
d synthetic mRNAs encoding ZP1, ZP2, and ZP3 proteins were injected into th
e cytoplasm of stage VI Xenopus oocytes. After 20 h of incubation the oocyt
es were harvested, and posttranslationally modified zona proteins were dete
cted with monoclonal antibodies specific to mouse ZP1, ZP2, and ZP3. The oo
cytes were imaged with confocal microscopy to detect individual zona protei
ns in the extracellular matrix of the oocytes, and this localization was co
nfirmed biochemically, Thus the mouse zona proteins appear to have been suf
ficiently conserved through 350 million years of evolution to be incorporat
ed into the extracellular envelope surrounding Xenopus eggs.