Molecular basis for oviductin-mediated processing from gp43 to gp41, the predominant glycoproteins of Xenopus egg envelopes

Acquisition of fertilizability in Xenopus coelomic eggs is correlated with the conversion from coelomic to vitelline envelope during passage of the eg gs through the pars recta portion of oviduct. The conversion includes proce ssing of a major envelope constituent gp43 of coelomic envelopes to gp41 of vitelline envelopes by a trypsin-type protease, oviductin, which is secret ed from the pars recta. Our recent sequencing analyses [Kubo et al., (1997) : Dev Growth Diff 39:405-411] strongly suggested that the N-terminal portio n of gp41 is exposed as a result of oviductin digestion. In this study, a m onoclonal antibody specific to the predicted N-terminus of gp41 was raised by immunizing mice with a synthetic N-terminal hexapeptide (QLPVSP) coupled to keyhole limpet hemocyanin. The antibody specifically reacted to gp41, b ut not to gp43, indicating that Gln62 is exposed as the N-terminal amino ac id of gp41 by oviductin-mediated cleavage of gp43 at Arg61 in GSR61. The C- terminal sequencing of gp43 and gp41 indicated that Arg373 in GSR373 as the C-terminus of gp41 is generated by cleavage of three amino acid (WNQ) resi dues from the C-terminus of gp43. The resulting polypeptide moiety of gp41 has a molecular mass of 33900 Da with 312 amino acid residues. We propose t hat oviductin possessing the substrate specificity of GSR simultaneously di gests gp43 at Arg residues in GSR61 and GSR373 to generate the N- and C-ter minus of gp41, respectively. Dev. Genet. 25:123-129, 1999. (C) 1999 Wiley-L iss, Inc.