Glycoprotein Hormone Assembly in the Endoplasmic Reticulum

Abstract

The unique structures of human choriogonadotropin (hCG) and related glycoprotein hormones make them well suited for studies of protein folding in the endoplasmic reticulum. hCG is stabilized by a strand of its β-subunit that has been likened to a “seatbelt” because it surrounds α-subunit loop 2 and its end is “latched” by an intrasubunit disulfide bond to the β-subunit core. As shown here, assembly begins when parts of the NH2 terminus, cysteine knot, and loops 1 and 3 of the α-subunit dock reversibly with parts of the NH2 terminus, cystine knot, and loop 2 of the hCG β-subunit. Whereas the seatbelt can contribute to the stability of the docked subunit complex, it interferes with docking and/or destabilizes the docked complex when it is unlatched. This explains why most hCG is assembled by threading the glycosylated end of α-subunit loop 2 beneath the latched seatbelt rather than by wrapping the unlatched seatbelt around this loop. hCG assembly appears to be limited by the need to disrupt the disulfide that stabilizes the small seatbelt loop prior to threading. We postulate that assembly depends on a “zipper-like” sequential formation of intersubunit and intrasubunit hydrogen bonds between backbone atoms of several residues in the β-subunit cystine knot, α-subunit loop 2, and the small seatbelt loop. The resulting intersubunit β-sheet enhances the stability of the seatbelt loop disulfide, which shortens the seatbelt and secures the heterodimer. Formation of this disulfide also explains the ability of the seatbelt loop to facilitate latching during assembly by the wraparound pathway.