Influence of cyclic nucleotides on the processing of the carbohydrate part of the alpha-subunit of human choriogonadotropin by first trimester human placenta tissue

Rapid Maturation of Glycoprotein Hormone Free α-Subunit (GPHα) and GPHαα Homodimers

The dynamics of glycoprotein hormone α-subunit (GPHα) maturation and GPHαα homodimer formation were studied in presence (JEG-3 choriocarcinoma cells) and absence (HeLa cells) of hCGβ. In both cases, the major initially occurring GPHα variant in [35S]Met/Cys-labeled cells carried two N-glycans (Mr app = 22 kDa). Moreover, a mono-N-glycosylated in vivo association-incompetent GPHα variant (Mr app = 18 kDa) was observed. In JEG-3 cells the early 22-kDa GPHα either associated with hCGβ, or showed self-association to yield GPHαα homodimers, or was later converted into heavily glycosylated large free GPHα (Mr app = 24 kDa). Micro-preparative isolation of intracellular GPHαα homodimers of JEG-3 cells and their conversion by reduction revealed that they consisted of 22-kDa GPHα monomers and not of large free GPHα. In HeLa cells, the large free GPHα variant was not observed, whereas GPHαα homodimers were present. Intracellularly, early GPHαα homodimers (35 kDa) and late variants (JEG-3: 44 kDa, HeLa: 39 kDa) were found. Both cell types secreted 45 kDa GPHαα homodimers. Large free GPHα and GPHαα homodimers were more rapidly sialylated than hCG αβ-heterodimers indicating a sequestration mechanism in the secretory pathway. In GPHαα homo- as well as hCG αβ-heterodimers the subunit interaction site, located on loop 2 of GPHα (amino acids 33–42), became immunologically inaccessible indicating similar spatial orientation of GPHα in both types of dimers. The studies demonstrate the formation, in vivo dynamics of GPHαα homodimers, and the pathways of the cellular metabolism of variants of GPHα, monoglycosylated GPHα and large free GPHα.

Disulfide bond formation is not required for human chorionic gonadotropin subunit association. Studies with dithiothreitol in JEG-3 cells

To study the influence of disulfide bridge formation on the assembly of the subunits of human chorionic gonadotropin in JEG-3 choriocarcinoma cells, dithiothreitol (DTT) was used to create a reducing milieu in the endoplasmic reticulum (ER) in vivo. In the presence of 5 mM DTT during pulse-chase experiments all of the beta-subunit precursors observed in unperturbed cells (pbeta(0), pbeta(1), pbeta(2), and beta(*)) collapsed into the pbeta(0) form. The reducing milieu of the ER was reoxidized in less than 5 min after removal of DTT from the medium. DTT markedly increased the half-life of the pbeta(0) precursor from 8.8 to 65.2 min. Under reoxidation conditions, the beta-subunit precursors folded back from pbeta(0) in less than 5 min. In unperturbed JEG-3 cells, the alpha-subunit was present in both fully glycosylated and monoglycosylated precursor (pre-alpha) forms. The attachment of the second N-linked glycan residue of the alpha-subunit was accelerated in the presence of DTT, and consequently pre-alpha-subunit was missing from the DTT-treated cultures. The formation of alphabeta-dimers appeared to be at least partially independent of the oxidation state in the ER. The alphabeta-dimer was present under conditions in which disulfide bridge formation was prevented by exposure to 5 mM DTT before and during the pulse period. This clearly suggests that the human chorionic gonadotropin subunits may acquire association-competent conformations even when no disulfide bridge formation has taken place.