THE ASPARGINE-LINKED OLIGOSACCHARIDES OF THE HUMAN CHORIONIC-GONADOTROPIN BETA-SUBUNIT FACILITATE CORRECT DISULFIDE BOND PAIRING

The role of asparagine (N)-linked oligosaccharide chains in intracellular folding of the human chorionic gonadotropin (hCG)-beta subunit was determined by examining the kinetics of folding in Chinese hamster ovary (CHO) cells transfected with wild-type or mutant hCG-beta genes lacking one or both of the asparagine glycosylation sites, The half time for folding of p beta 1 into p beta 2, the rate-determining step in beta folding, was 7 min for wildtype beta but 33 min for beta lacking both N-linked glycans, The p beta 1 --> p beta 2 half-time was 7.5 min in CHO cells expressing the beta subunit missing the Asn(13)-linked glycan and 10 min for the beta subunit missing the Asn(30)-linked glycan. The inefficient folding of hCG-beta lacking both N linked glycans correlated with the slow formation of the last three disulfide bonds (i,e, disulfides 23-72, 93-100, and 26-110) to form in the hCG-beta-folding pathway. Unglycosylated hCG-beta was slowly secreted from CHO cells, and beta subunit-folding intermediates retained in cells for more than 5 h were degraded into a hCG-beta core fragment-like protein, However, coexpression of the hCG-alpha gene enhanced folding and formation of disulfide bonds 23-72, 93-100, and 26-110 of hCG-beta lacking N-linked glycans, In addition, the molecular chaperones BiP, ERp72, and ERp94, but not calnexin, were found in a complex with unglycosylated, unfolded hCG-beta and may be involved in the folding of this beta form, These data indicate that N-linked oligosaccharides assist hCG-beta subunit folding by facilitating disulfide bond formation.