Identification of a region of mouse zona pellucida glycoprotein mZP3 that possesses sperm receptor activity

The ability of mouse zona pellucida glycoprotein ZP3 (mZP3) to function as a sperm receptor is attributable to certain of its oligosaccharides, not to its polypeptide (P. M. Wassarman, 1990. Development108, 1-17). Here, purified, radioiodinated mZP3 was digested by either papain or V8 protease, and the glycopeptides produced were fractionated by HPLC and assayed for sperm receptor activity in vitro. Each proteolytic digest of mZP3 contained a heavily glycosylated peptide, ∼55,000 apparent M_r, that exhibited sperm receptor activity in vitro. To determine the region of mZP3 polypeptide from which the active glycopeptides were derived, Western gel immunoblotting, employing an antiserum directed against a specific mZP3 peptide epitope, and automated amino-terminal amino acid sequencing were employed. Results of these experiments strongly suggest that the active glycopeptides produced by digestion of mZP3 with either papain or V8 protease are derived from the same region of the carboxy-terminal half of the mZP3 polypeptide. These and other findings are discussed in terms of mZP3 structure and function.