Characterization of two zebrafish cDNA clones encoding egg envelope proteins ZP2 and ZP3

Two zebrafish cDNA clones encoding homologs of mammalian zona pellucida proteins ZP2 and ZP3 were isolated from a whole adult cDNA library. The ZP2 clone encodes a protein of 428 amino acids. Unlike other teleost ZP2s that contain an N-terminal repetitive domain enriched with prolines and glutamines, the zebrafish ZP2 has no such repetitive domain. In the C-terminal non-repetitive domain, the zebrafish ZP2 shares 55-76% sequence identity with other teleost ZP2s. The ZP3 cDNA clone encodes a protein of 431 amino acids, which shares 61% sequence identity with a carp ZP3. Similar to mammalian ZP proteins, both zebrafish ZP2 and ZP3 contain several potential phosphorylation sites. However, unlike mammalian ZP proteins, both zebrafish ZP proteins contain almost no glycosylation site, which has been proposed to be important for interaction with sperm; thus, the ZP proteins may behave differently in mammals and teleosts. Northern blot analysis indicated that both zebrafish ZP2 and ZP3 mRNAs were expressed exclusively in the ovary and hence the ovary is likely the only site for ZP2 and ZP3 biosynthesis.