Post-translational requirements for functional factor V and factor VIII secretion in mammalian cells

Coagulation factors V and VIII are homologous glycosylated plasma proteins that provide essential functions for hemostasis. Factor V is secreted as a single chain polypeptide, whereas factor VIII is processed intracellularly to yield a metal-ion-associated heterodimer that is stabilized through interaction with von Willebrand factor. In transfected mammalian cells, factor V is more efficiently secreted than factor VIII. To provide insight into the different secretion efficiencies, we compared the post-translational processing requirements for factor V and factor VIII expressed in mammalian cells. In contrast to factor VIII, factor V was not detected in association with the immunoglobulin-binding protein (BiP), a chaperonin protein of the endoplasmic reticulum (ER). Depletion of intracellular ATP levels by treatment of cells with low concentrations of carbonyl cyanide 3-chlorophenylhydrazone (CCCP), protonophore that uncouples oxidative phosphorylation, inhibited secretion of factor VIII but had no effect on the secretion of factor V. Inhibition of N-linked oligosaccharide addition by treatment with tunicamycin prevented secretion of both factor V and factor VIII, whereas treatment with an inhibitor of complex oligosaccharide addition, deoxymannojirimycin, did not affect secretion, although the specific activities of both factor V and factor VIII were slightly increased. Thus, complex oligosaccharide addition was not required for secretion or functional activity of either factor V or factor VIII. Depletion of intralumenal calcium with the ionophore A23187 did not affect secretion of either factor V or factor VIII. In the presence of A23187, the secreted factor V was fully functional, whereas the factor VIII heavy and light chains were not associated and the secreted molecule was inactive. In addition, A23187 treatment inhibited addition of serine/threonine (O)-linked oligosaccharides to factor V and factor VIII. The differences between factor V and factor VIII were further evaluated by characterization of a single chain mutant factor VIII. The single chain factor VIII was secreted with an efficiency similar to wild-type factor VIII and also required von Willebrand factor for stabilization. In addition, the activity of single chain factor VIII was also inhibited by A23187 treatment, suggesting a unique metal-ion requirement within the secretory pathway for functional factor VIII folding. The differences identified in BiP association, ATP requirements, and metal-ion dependence for effective functional secretion of these two molecules may underlie mechanisms accounting for their different secretion efficiencies.