Genomic structure, and expression of zona pellucida genes of winter flounder and zebra fish

The teleost egg is surrounded by an extracellular matrix, the zona pellucida, which is composed of a network of protein fibers deposited in a regular arrangement. The zona pellucida is first line of defense against polyspermy. After fertilization, the proteins of the zona pellucida undergo a crosslinking reaction which results in hardening and insolubility of the zona pellucida. During the development of the embryo, the zona pellucida provides mechanical protection, while allowing exchange of metabolites with the outer environment. The biochemistry of the zona pellucida has been studied in various teleost species, and it has been found to comprise one to four major protein species. Our laboratory was first to examine the Zp genes in winter flounder (WF) during oogenesis. WF Zp alpha gene consists of eight exons separated by seven introns of different sizes. Within exon 1, there are six PQQ-rich repeats. Four of them encode a putative (PQQ) sub(4)PKY polypeptide, similar to the repeats found in the extracellular domains of other proteins. Exons 2-7 share homology with the zona pellucida protein genes rc55 of rabbit and zp- alpha of mouse, and the positions of intron boundaries are conserved in the WF and mouse zp- alpha genes. Three Zp proteins (Zp alpha , Zp beta and Zp gamma ) have been isolated from WF zona pellucida by gel filtration chromatography (Sepharose CL-6B) using urea solubilized Zp extracts. Internal sequences of several tryptic peptides of WF Zp alpha protein are identical to those deduced from DNA sequence of the cloned Zp alpha gene. Using the Zp alpha cDNA of goldfish, we have recently isolated the zebra fish Zp alpha gene. While Zp alpha is primarily expressed in liver of vitollogenic female WF, Zp alpha is expressed in both ovary and liver of zebrafish. The role of Zp, especially the function of PQQ-rich domain of the Zp protein which is unique to the teleosts is currently being investigated.