A membrane-anchored form but not the secretory form of human chorionic gonadotropin-alpha chain acquires polylactosaminoglycan.

Large polylactosaminoglycans have only been observed linked to membrane proteins. To determine if membrane anchoring of a secretory protein might lead to the addition of polylactosaminoglycan, we have examined the carbohydrate structure on a membrane-anchored form of human chorionic gonadotropin-alpha subunit. This protein was generated by fusing the DNA encoding the human chorionic gonadotropin-alpha subunit to the DNA encoding the membrane-spanning and cytoplasmic domains of the vesicular stomatitis virus glycoprotein. DNAs encoding this hybrid form and the secretory form of human chorionic gonadotropin-alpha were expressed in monkey COS-1 cells using an SV40-based vector. We show here that the parent secretory glycoprotein contains typical Asn-linked complex-type oligosaccharides while the membrane-bound form contains large, heterogenous polylactosaminoglycans. We conclude that membrane anchoring increases the accessibility of the N-linked glycans to the enzymes involved in polylactosamine addition. The inhibitor 1-deoxymannojirimycin blocks addition of the polylactosaminoglycan.