Size variations in the mucin-type domain of hamster oviductin: identification of the polypeptide precursors and characterization of their biosynthetic maturation

The recent sequencing of a cDNA clone for hamster oviductin revealed that this zona pellucida-associated glycoprotein is a particularly intriguing chimeric molecule because it encloses regions of significant similarity with chitinase-related proteins as well as a carboxyterminal mucin-type domain. This domain contains contiguous Ser/Thr-rich repeated stretches of 15 amino acids; similar units are also found in the deduced sequence of human oviductin. Such structural domains constitute a central feature of mucins. We amplified this region from 16 hamster oviductin cDNA clones and identified three length variants. In order to elucidate the biosynthetic maturation of the glycoprotein, a high-titer antiserum against synthetic peptides derived from internal sequences of hamster oviductin was produced and used in pulse-chase experiments. Two major and one minor polypeptide precursors were identified from tunicamycin-treated cell lysates and in vitro translated products from oviductal poly(A)+ RNA. Their apparent molecular masses correlate with the predicted lengths of the three size variants identified by polymerase chain reaction (PCR) amplification. Using glycosylation and transport inhibitors, we sought to dissect the posttranslational sequential steps leading to the final maturation of hamster oviductin and proposed a compartmental model for its biosynthesis. The polypeptide precursors are rapidly converted in the endoplasmic reticulum into an N- and O-glycosylated premature form of 80-90 kDa (time < 20 min), which is further O-glycosylated and sulfated in the trans-Golgi network, giving rise to the secreted species of 160-350 kDa. The polymorphism in the heavily O-glycosylated region of hamster oviductin is predicted to increase the heterogeneity of the glycoprotein. Such changes may alter the putative biological function of the different variants mediated by their mucin-type domain, such as protection of the embryo and/or adhesion-related phenomena.