Posttranslational modification of the carboxy-terminal region of the beta subunit of human chorionic gonadotropin.

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The beta subunit of human chorionic gonadotropin (hCG) contains at its carboxy terminus an extension of 29 amino acids not found in the beta subunits of the other glycoprotein hormones. This region provides the sites of attachment of four serine-linked oligosaccharide chains. We have examined the synthesis of this subunit in a cell-free translation system derived from Krebs II ascites tumor cells. The primary translation product was found to undergo a temperature-dependent posttranslational modification which resulted in an increase in apparent molecular weight of 2000 on sodium dodecyl sulfate gel electrophoresis. This modification was specific for the beta subunit of hCG, since no changes were observed for the beta subunit of bovine luteinizing hormone or for the alpha subunits of either hormone. The increase in molecular weight occurred in the absence of microsomal membranes and was not due to the addition of N-linked carbohydrate. An identical shift was observed when pre-hCG beta was incubated with extracts of human placenta. The site of modification was localized by fingerprint analysis to a carboxy-terminal tryptic peptide which contains two of the four O-glycosylated serine residues in the mature form of the subunit. The modified protein was resistant to oligosaccharidase digestion and beta-elimination, indicating that it does not contain O-linked oligosaccharides of the type found on mature hCG beta. These results demonstrate that a specific modification of the carboxy-terminal segment of hCG beta synthesized in vitro occurs in the absence of O-linked glycosylation.