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

Abstract

The .beta. subunit of human chorionic gonadotropin (hCG) contains as 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 4 serine-linked oligosaccharides chains. The synthesis of this subunit was examined 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 MW 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 MW 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 2 of the 4 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.. A specific modification of the carboxy-terminal segment of hCG .beta. synthesized in vitro occurs in the absence of O-linked glycosylation.