Primary structure and developmental expression of Dp ZP2, a vitelline envelope glycoprotein homolog of mouse ZP2, in Discoglossus pictus, one of the oldest living anuran species

A glycoprotein of the Xenopus vitelline envelope, gp 69/64, which mediates sperm binding, is closely related to the components of ZPA family, such as the mouse zona pellucida ZP2. To test the generality of these findings, we studied Discoglossus pictus, a species evolutionary distant from Xenopus an d identified as a protein of 63 kDa in the vitelline envelope. Preliminary studies suggest that this protein may bind sperm at fertilization. We found that the 63-kDa protein is glycosylated and contains both N- and O-linked chains. We have cloned the cDNA encoding the Discoglossus protein of 63 kDa (Dp ZP2) by screening a Discoglossus cDNA library using Xenopus gp 69/64 c DNA as a probe. Analysis of the deduced sequence of Discoglossus protein re vealed 48% identity with Xenopus gp 69/64 and 37-40% identity with mouse ZP 2. The sequence conservation included a ZP domain, a potential furin cleava ge site and a putative transmembrane domain. The N-terminus region of Dp ZP 2 was 40% identical to the corresponding region of Xenopus gp 69/64 which h as been shown to be essential for sperm binding to the VE. Although, as of yet, there is no evidence for sperm binding at the Dp ZP2 N-terminus, it is interesting that in this region three potential O-glycosylation sites are conserved in both species, in contrast to N-glycosylation sites. It was fou nd that the Dp ZP2 mRNA is expressed in stage 1 oocytes and in the follicle cells surrounding the oocyte. Similarly, in Xenopus oocytes, the go 69/64m RNA, was found in the oocytes, as well as in the somatic cells. Mol. Repro d. Dev. 59: 133-143, 2001. (C) 2001 Wiley-Liss, Inc.