Es. Litscher et al., Mouse zona pellucida glycoproteins mZP2 and mZP3 undergo carboxy-terminal proteolytic processing in growing oocytes, BIOCHEM, 38(38), 1999, pp. 12280-12287
The extracellular coat, or zona pellucida, of the mouse egg consists of thr
ee glycoproteins, called mZP1-3. The glycoproteins are synthesized and secr
eted concomitantly by growing oocytes during their 2-3-week growth phase. E
ach of the glycoproteins has a consensus furin cleavage site (-Arg-X-Lys/Ar
g-Arg-) near the C-terminus of their polypeptide. Here, several approaches
were employed to determine whether nascent mZP2 and mZP3 are cleaved at the
consensus sites, -Arg-Ser-Lys-Arg- and -Arg-Asn-Arg-Arg-, respectively, pr
ior to secretion. Molecular mass determinations of deglycosylated mZP2 and
mZP3 suggest that their polypeptides are similar to 9 and similar to 7 kDa
smaller, respectively, than predicted from exon sequences. Two-dimensional
thin-layer chromatographic analyses were also carried out to identify amino
acids released from the C-terminus of mZP2 and mZP3 by carboxypeptidase B.
On the basis of exon sequences, there are no Arg residues at the predicted
C-terminus of the mature glycoproteins. However, for both mZP2 and mZP3, A
rg residues were released by carboxypeptidase B, consistent with processing
at the consensus furin cleavage site. Furthermore, an antiserum raised aga
inst an mZP3 peptide, located downstream of the consensus furin cleavage si
te, failed to label purified mZP3 on Western immunoblots. The antiserum als
o failed to label the zona pellucida of oocytes examined by laser scanning
confocal microscopy. Collectively, these results strongly suggest that mZP2
and mZP3 are processed at their consensus furin cleavage site prior to sec
retion and incorporation into the zona pellucida.