Oviductin, the Xenopus laevis Oviductal Protease That Processes Egg Envelope Glycoprotein gp43, Increases Sperm Binding to Envelopes, and Is Translated as Part of an Unusual Mosaic Protein Composed of Two Protease and Several CUB Domains1

Abstract

The glycoprotein envelope surrounding the Xenopus laevis egg is converted from an unfertilizable to a fertilizable form during transit through the pars recta portion of the oviduct. Envelope conversion involves the pars recta protease oviductin, which selectively hydrolyzes envelope glycoprotein gp43 to gp41. Oviductin cDNA was cloned, and sequence analysis revealed that the protease is translated as the N terminus of an unusual mosaic protein. In addition to the oviductin protease domain, a protease domain with low identity to oviductin was present, possessing an apparent nonfunctional catalytic site. Three CUB domains were also present, which are related to the mammalian spermadhesin molecules implicated in mediating sperm-envelope interactions. We propose that during post-translational proteolytic processing of the mosaic oviductin glycoprotein, the processed N-terminal protease domain is released coupled to two C-terminal CUB domains and constitutes the enzymatically active protease molecule. In functional studies, isolated coelomic egg envelopes treated with oviductin purified from the oviduct showed a dramatic increase in sperm binding. This observation established that oviductin alone was the oviductal factor responsible for converting the egg envelope to a sperm-penetrable form, via an increase in sperm binding. Trypsin mimicked oviductin's effect on envelope hydrolysis and sperm binding, demonstrating that gp43 processing is the only requirement for envelope conversion.

Human Polyserase-2, a Novel Enzyme with Three Tandem Serine Protease Domains in a Single Polypeptide Chain

“…By contrast, polyserase-1 undergoes a complex series of proteolytic events that lead to the generation of three independent serine protease units (10). Likewise, amphibian oviductin and ovochymase also release their respective protease modules after translation of a single polyprotein product (11)(12)(13). Structural analysis of the sequences preceding the catalytic domains of these modules may provide an explanation for the observed inability of polyserase-2 to generate independent protease units.…”

“…This is the case for ovochymase from Xenopus laevis, which also possesses three serine protease modules that are post-translationally released from a single polyprotein product (11). Likewise, oviductin from X. laevis or Bufo japonicus contains two serine protease domains that also undergo proteolytic processing events to generate independent units (12,13). Furthermore, analysis of the genome sequence of D. melanogaster and C. elegans has also revealed the occurrence of some predicted genes with several serine protease modules, although it is not known whether they are an integral part of a single polyprotein or are post-translationally cleaved (14,15).…”

Polyserase-I, a human polyprotease with the ability to generate independent serine protease domains from a single translation product

“…This situation, albeit extremely unusual, is not unprecedented in other organisms and resembles that recently described for ovochymase from Xenopus laevis, another complex mosaic protein with three serine protease domains that are released after translation of a single polyprotein product (32). Likewise, oviductin from Xenopus laevis or Bufo japonicus (33,34) contains two serine protease domains, one of them catalytically inactive, that also undergo posttranslational proteolytic processing events to generate independent units. Both ovochymase and oviductin are polyproteases involved in fertilization processes, although they do not contain transmembrane domains and therefore do not belong to the TTSP family, as is the case of polyserase-I.…”

Molecular Cloning and Characterization of Glucanase Inhibitor Proteins

“…This apparently is the case with GIPs and other catalytically inactive Ser protease homologs, which exhibit a wide range of biological functions but which share a unifying theme of high-affinity protein-protein recognition (Kurosky et al, 1980;Isackson and Bradshaw, 1984;Højrup et al, 1985;Nakamura et al, 1989;Lindsay et al, 1999;MurugasuOei et al, 1995;Lindsay et al, 1999;Huang et al, 2000). However, to our knowledge, GIPs represent the first example of proteolytically inactive Ser protease homologs that function as enzyme inhibitors.…”