Molecular chaperones facilitate the folding of proteins in the endoplasmic reticulum (ER) of mammalian cells, The glycoprotein hormone chorionic gonadotropin beta 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-beta subunit occurs in the ER with a t(1/2) = 4-5 min, stable association of ER chaperones with hCG-beta have been difficult to detect probably because they have a short half-life, However, beta-chaperone complexes containing the ER chaperones BiP, ERp72, and ERp94 have been detected in slow folding mutants of hCG-beta 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-beta subunit folding, 2) whether such complexes are dead-end or whether they lead to formation of a secreted, mature hCG-beta form, and 3) what the nature of the hCG-beta-chaperone binding is, The data obtained indicate that the amount of detectable hCG-beta-chaperone complexes correlates with the rate or extent of folding, that the complexes of hCG-beta with ER chaperones lead to the formation of secretable beta, and that the complexes of hCG-beta with chaperones involve the formation of intermolecular disulfide bonds.
