Cluster of genes encoding the major egg envelope protein of zebrafish

The proteins of fish egg envelopes are encoded by genes that are closely related to the genes for human zona pellucida proteins. A cluster of three genes coding for an egg envelope protein was isolated from the zebrafish, Danio rerio. The three genes, zp2a, zp2b, and zp2c, are located within an 11 kb region and are each comprised of eight exons spanning 1.85 kb. The exon-intron structures of the genes are nearly identical; however, their deduced amino acid sequences diverge at exon 7 (zp2b and zp2c from zp2a) and exon 8 (zp2c from zp2b). Exons 2-7 have a structural organization similar to exons in the carboxy-terminal half of the human zona pellucida ZP1, ZP2, and ZPB genes, suggesting they arose from a common ancestral gene. Sequence comparisons indicate that the deduced zebrafish proteins are most closely related to human ZPB. Zebrafish mRNAs coding for each of the three ZP2 variants have been found either as full-length cDNAs or expressed sequence tags. Distinct from the wf(female) gene of winter flounder which we first reported (Lyons et al., 1993: J Biol Chem 268:21351-21358), expression of the zebrafish zp2 genes was found to be ovary-specific, instead of liver-specific, and the promoter regions of zp2a and zp2b, while different, both contained E-box sequences (CANNTG) that have been demonstrated to be essential for coordination of zona pellucida gene expression in mammalian oocytes. Mixed peptide sequence analysis was used to identify the major polypeptide component of isolated zebrafish egg envelopes as the zp2 gene product.