Nonassembled human chorionic gonadotropin subunits and alphaalpha-homodimers use fast-track processing in the secretory pathway in contrast to alphabeta-heterodimers

In multimeric glycoproteins, like glycoprotein hormones, mutual subunit interactions are required for correct folding, assembly, and transport in the secretory pathway. However, character and time course of these interactions need further elucidation. The influence of the glycoprotein hormone alpha-subunit (GPHalpha) on the folding of the human chorionic gonadotropin (hCG) beta-subunit (hCGbeta) in hCG alphabeta-heterodimers was investigated in [(35)S]Met/Cys-labeled JEG-3 cells. Completeness of disulfide bridge formation during the time course of folding was estimated by labeling with [(3)H]N-ethylmaleinimide of free thiol groups not yet consumed. Subunit association took place between immature hCGbeta (high (3)H/(35)S ratio) and almost completely folded GPHalpha. Analysis revealed a highly dynamic maturation process comprising of at least eight main hCGbeta folding intermediates (molecular masses from 107 to 28 kDa) that could be micro-preparatively isolated and characterized. These hCGbeta variants developed while being associated with GPHalpha. The 107-kDa variant was identified as a complex with calnexin. In contrast to hCG alphabeta-heterodimers, free nonassociated hCGbeta, free large GPHalpha, and GPHalphaalpha homodimers showed a fast-track-like processing in the secretory pathway. At 10 min before hCG secretion, sialylation of these variants had already been completed in the late Golgi, whereas hCG alphabeta-heterodimers had still not arrived medial Golgi. This shows that the GPHalpha in the hCG alphabeta-heterodimers decelerates the maturation of the hCGbeta portion in the heterodimer complex. This results in a postponed approval of hCG alphabeta-heterodimers by the endoplasmic reticulum quality control unlike GPHalphaalpha homodimers, free hCGbeta, and GPHalpha subunits.