The recombinant equine LH beta subunit combines divergent intracellular traits of human LH beta and CG beta subunits

Abstract

The pituitary LH beta and placental CG beta 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 hLH beta is replaced by a longer O-glycosylated carboxyterminal peptide in hCG beta. 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 CG beta and LH beta subunits are products of the same gene expressed in the placenta and pituitary (LH beta), and both contain a carboxy-terminal peptide. This unusual expression pattern intrigued us and led to our study of eLH beta subunit secretion by transfected Chinese hamster ovary and Mad in-Darby canine kidney cells. In continuous labeling and pulse-chase experiments, the secretion of the eLH beta subunit from the transfected Chinese hamster ovary cells was inefficient (medium recovery of 16%-25%) and slow (t(1/2)>6.5 hours). This indicated that, the secretion of the eLH beta subunit resembles that of hLH beta rather than hCG beta. In Madin-Darby canine kidney cells grown on Transwell filters, the eLH beta subunit was preferentially secreted from the apical side, similar to the hCG beta subunit secretory route (similar to 65% of the total protein secreted). Taken together, these data suggested that secretion of the eLH beta subunit integrates features of both hLH beta and hCG beta subunits. We propose that the evolution of this intracellular behavior may fulfill the physiological demands for biosynthesis of the LH and CG beta-subunits in the pituitary and placenta, respectively.