Glycoprotein Hormone Assembly in the Endoplasmic Reticulum

Glycoprotein hormone heterodimers are stabilized by their unusual structures in which a glycosylated loop of the -subunit straddles a hole in the -subunit. This hole is formed when a cysteine at the end of a -subunit strand known as the “seatbelt” becomes “latched” by a disulfide to a cysteine in the -subunit core. The heterodimer is stabilized in part by the difficulty of threading the glycosylated end of the -subunit loop 2 through this hole, a phenomenon required for subunit dissociation. Subunit combination in vitro, which occurs by the reverse process, can be accelerated by removing the -subunit oligosaccharide. In cells, heterodimer assembly was thought to occur primarily by a mechanism in which the seatbelt is wrapped around the -subunit after the subunits dock. Here we show that this “wraparound” process can be used to assemble disulfide crosslinked human choriogonadotropin analogs that contain an additional -subunit cysteine, but only if the normal -subunit latch site has been removed. Normally, the seatbelt is latched before the subunits dock and assembly is completed when the glycosylated end of -subunit loop 2 is threaded beneath the seatbelt. The unexpected finding that most assembly of human choriogonadotropin, human follitropin, and human thyrotropin heterodimers occurs in this fashion, indicates that threading may be an important phenomenon during protein folding and macromolecule assembly in the endoplasmic reticulum. We suggest that the unusual structures of the glycoprotein hormones makes them useful for identifying factors that influence this process in living cells.