Summary
The vitamin K-dependent carboxylation of specific glutamyl residues to γ-carboxyglutamyl
residues occurs during the endoplasmic reticulum processing of a limited number of
proteins. The fate of the under-γ-carboxylated proteins during protein processing
was studied. When human hepatoma (HepG2) cells were grown in the presence of warfarin,
under-γ-carboxylated prothrombin was secreted into the medium. In contrast, prothrombin
secretion from a rat hepatoma (H-35) cell line was blocked by warfarin, and intracellular
forms which were retained were degraded. When rat prothrombin (rFII) was stably transfected
into warfarin treated HepG2 cells, endogenous human prothrombin (hFII) was secreted
in an under-γ-carboxylated form, while rFII accumulated intracellularly. These data
indicate that retention and degradation of under-γ-carboxylated prothrombin by human
hepato-cytes is related to a structural difference in rFII and hFII. When rFII and
hFII were transfected into a warfarin treated transformed human embryonic kidney cell
line (293), both proteins were secreted in an under-γ-carboxylated form and intracellular
retention was not observed. However, the secretion of rFII was greatly diminished.
Cellular retention of under-γ-carboxylated forms is therefore tissue specific, but
degradation is not.
