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 t hat this zona pel lucida-associated glycoprotein is a particularly int riguing chimeric molecule because it encloses regions of significant s imilarity with chitinase-related proteins as well as a carboxyterminal mucin-type domain. This domain contains contiguous Ser/Thr-rich repea ted stretches of 15 amino acids; similar units are also found in the d educed 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-t iter antiserum against synthetic peptides derived from internal sequen ces of hamster oviductin was produced and used in pulse-chase experime nts, Two major and one minor polypeptide precursors were identified fr om tunicamycin-treated cell lysates and in vitro translated products f rom oviductal poly(A)(+) RNA. Their apparent molecular masses correlat e 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 seque ntial steps leading to the final maturation of hamster oviductin and p roposed a compartmental model for its biosynthesis. The polypeptide pr ecursors 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, gi ving 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 alte r the putative biological function of the different variants mediated by their mucin-type domain, such as protection of the embryo and/or ad hesion-related phenomena.