Converts the glycoprotein envelope surrounding the egg from an unfertilizable to a fertilizable form during its transit through the pars recta portion of the oviduct by selectively hydrolyzing the envelope glycoprotein gp43. The egg envelope is converted to a sperm-penetrable form, via an increase in sperm binding. Preferential cleavage at 371-Gly-Ser-Arg-|-Trp-374 of glycoprotein gp43 in Xenopus laevis coelemic egg envelope to yield gp41. Specifically expressed in the pars recta oviduct. The catalytically inactive 110 kDa form is processed both N- and C-terminally to give rise to the 66 kDa catalytically active form. Belongs to the peptidase S1 family.