N-Glycosylation Contributes to the Intracellular Stability of Prothrombin Precursors in the Endoplasmic Reticulum

Abstract

Rat prothrombin (rFII) and human prothrombin (hFII) are processed differently during their biosynthesis in a manner dependent upon γ-glutamyl carboxylation, a vitamin K-dependent posttranslational modification of prothrombin precursors. The role of N-glycosylation in the cellular processing of prothrombin was examined in hepatoma (H-35 and HepG2) and transformed kidney (HEK293) cell lines. Aglyco-rFII obtained by tunicamycin treatment was degraded in warfarin-treated H-35 cells, but not in vitamin K-treated cells. Fully glycosylated rFII is also selectively retained and degraded in warfarin-treated H-35 cells. When rFII and hFII were transiently expressed in tunicamycin-treated HEK293 cells, rFII but not hFII was degraded to generate a 48-KD species. This degradation was independent of γ-carboxylation, indicating that the sensitivity of aglyco-rFII and aglyco-hFII toward this specific proteolysis differs in HEK293 cells. By expressing chimeric rFII/hFII constructs in tunicamycin-treated HEK293 cells, it was shown that the kringle 2 structure of prothrombin was responsible for this difference. The 48-KD species was not observed in tunicamycin-treated H-35 cells, suggesting that this specific proteolytic processing of aglyco-rFII is also cell-type specific. Expression of rFII in other tunicamycin-treated nonhepatic cells suggests that this cell-specific difference in processing might be determined by a difference between hepatic and nonhepatic cells. The site-directed mutagenesis utilized in these studies also establishes that the N-glycosylation sites of rFII are at residues Asn77, 101, 378, and 518.