Novel Covalent Chaperone Complexes Associated with Human Chorionic Gonadotropin β Subunit Folding Intermediates

Molecular chaperones facilitate the folding of proteins in the endoplasmic reticulum (ER) of mammalian cells. The glycoprotein hormone chorionic gonadotropin β subunit is a secretory protein whose folding in the ER has been demonstrated (Huth, J. R., Mountjoy, K., Perini, F., and Ruddon, R. W. (1992) J. Biol. Chem. 267, 8870-8879). Because folding of wild type hCG-β subunit occurs in the ER with a t1/2 = 4-5 min, stable association of ER chaperones with hCG-β have been difficult to detect probably because they have a short half-life. However, β-chaperone complexes containing the ER chaperones BiP, ERp72, and ERp94 have been detected in slow folding mutants of hCG-β subunit that lack both of the N-linked oligosaccharides (Feng, W., Matzuk, M. M., Mountjoy, K., Bedows, E., Ruddon, R. W., and Boime, I. (1995) J. Biol. Chem. 270, 11851-11859). The questions addressed here are 1) whether the detection of chaperone-containing complexes is related to the absence of carbohydrate or to the rate of hCG-β subunit folding, 2) whether such complexes are dead-end or whether they lead to formation of a secreted, mature hCG-β form, and 3) what the nature of the hCG-β-chaperone binding is. The data obtained indicate that the amount of detectable hCG-β-chaperone complexes correlates with the rate or extent of folding, that the complexes of hCG-β with ER chaperones lead to the formation of secretable β, and that the complexes of hCG-β with chaperones involve the formation of intermolecular disulfide bonds.