Time-dependent folding of immunological epitopes of the human chorionic gonadotropin β-subunit

We have explored the possibility to use 14 different monoclonal antibodies in order to follow the formation of the respective epitopes during the biosynthesis of hCG subunits and their association in JEG-3 choriocarcinoma cells using pulse (30 s to 5 min)-chase (0-180 min) experiments. We found central cystine knot epitope structures (epitope β1) to be formed immediately and simultaneously with epitopes on the protruding hCG-β loops 1 and 3. We found also differences in the time-dependent folding of β2 and β4 epitopes, which are highly overlapping structures on the loops 1 + 3. These differences were reinforced by decreasing the temperature during the pulse-chase experiments to 25 °C. Moreover, we describe for the first time an intracellular intact hCG β-subunit form that showed the transient expression of the hCG-β-core fragment epitope β11 in the course of the maturation of this subunit which casts new light on the presence of hCG-β-core fragment in Down's syndrome, tumors and pregnancy.