Abstract
The pituitary LHβ and placental CGβ subunits are products of different genes in primates. The major structural difference between the two subunits is in the carboxy-terminal region, where the short carboxyl sequence of hLHβ is replaced by a longer O-glycosylated carboxy-terminal peptide in hCGβ. In association with this structural deviation, there are marked differences in the secretion kinetics and polarized routing of the two subunits. In equids, however, the CGβ and LHβ subunits are products of the same gene expressed in the placenta and pituitary (LHβ), and both contain a carboxy-terminal peptide. This unusual expression pattern intrigued us and led to our study of eLHβ subunit secretion by transfected Chinese hamster ovary and Madin-Darby canine kidney cells. In continuous labeling and pulse-chase experiments, the secretion of the eLHβ subunit from the transfected Chinese hamster ovary cells was inefficient (medium recovery of 16%-25%) and slow (t1/2 > 6.5 hours). This indicated that, the secretion of the eLHβ subunit resembles that of hLHβ rather than hCGβ. In Madin-Darby canine kidney cells grown on Transwell filters, the eLHβ subunit was preferentially secreted from the apical side, similar to the hCGβ subunit secretory route (∼65% of the total protein secreted). Taken together, these data suggested that secretion of the eLHβ subunit integrates features of both hLHβ and hCGβ subunits. We propose that the evolution of this intracellular behavior may fulfill the physiological demands for biosynthesis of the LH and CG β-subunits in the pituitary and placenta, respectively.