Nonassembled Human Chorionic Gonadotropin Subunits and αα-Homodimers Use Fast-Track Processing in the Secretory Pathway in Contrast to αβ-Heterodimers

Abstract

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 α-subunit (GPHα) on the folding of the human chorionic gonadotropin (hCG) β-subunit (hCGβ) in hCG αβ-heterodimers was investigated in [35S]Met/Cys-labeled JEG-3 cells. Completeness of disulfide bridge formation during the time course of folding was estimated by labeling with [3H]N-ethylmaleinimide of free thiol groups not yet consumed. Subunit association took place between immature hCGβ (high 3H/35S ratio) and almost completely folded GPHα. Analysis revealed a highly dynamic maturation process comprising of at least eight main hCGβ folding intermediates (molecular masses from 107 to 28 kDa) that could be micro-preparatively isolated and characterized. These hCGβ variants developed while being associated with GPHα. The 107-kDa variant was identified as a complex with calnexin. In contrast to hCG αβ-heterodimers, free nonassociated hCGβ, free large GPHα, and GPHαα 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 αβ-heterodimers had still not arrived medial Golgi. This shows that the GPHα in the hCG αβ-heterodimers decelerates the maturation of the hCGβ portion in the heterodimer complex. This results in a postponed approval of hCG αβ-heterodimers by the endoplasmic reticulum quality control unlike GPHαα homodimers, free hCGβ, and GPHα subunits.