TREATMENT OF XENOPUS-LAEVIS CELOMIC EGGS WITH TRYPSIN MIMICS PARS RECTA OVIDUCTAL TRANSIT BY SELECTIVELY HYDROLYZING ENVELOPE GLYCOPROTEIN GP43, INCREASING SPERM BINDING TO THE ENVELOPE, AND RENDERING EGGS FERTILIZABLE

Xenopus laevis coelomic (body cavity) eggs are not fertilizable until they pass through the pars recta oviduct. A secreted pars recta oviduc tal protease with trypsin-like activity, oviductin, selectively hydrol yzes egg envelope glycoprotein gp43 to gp41; this limited proteolysis is believed to render the egg fertilizable. The effects of trypsin as a substitute for oviductin in modifying envelope structure and functio n were examined. Trypsinolysis (5 mIU for 30 min at room temperature) selectively converted gp43 to gp41 without hydrolysis of other envelop e glycoproteins, and rendered coelomic eggs fertilizable in the presen ce of a jelly water preparation. Chymotrypsin had no effect on the acq uisition of fertilizability, indicating that the reaction was dependen t on trypsin-like specificity. This was confirmed by the use of p-amin obenzamindine and leupeptin to inhibit the ability of trypsin preparat ions to induce fertilizability. A sperm binding assay revealed that tr ypsin treatment dramatically increased sperm binding to egg envelopes derived from both coelomic eggs and ovarian eggs. Jelly water was not required for sperm binding. Therefore, trypsin can mimic the biologica l action of oviductin, selectively cleaving egg envelope gp43 to gener ate or expose sperm binding sites, rendering the envelope penetrable b y sperm and permitting fertilization. (C) 1998 Wiley-Liss, Inc.