Biosynthesis of the subunits of factor VIIIR by bovine aortic endothelial cells

The biosynthesis of the subunit of factor VIIIR was studied in bovine aortic endothelial cells by the techniques of immunoprecipitation and NaDodSO4/polyacrylamide gel electrophoresis. It was determined that the subunit is first produced as a Mr 240,000 glycoprotein precursor, which appears to undergo proteolytic cleavage at or about the time of secretion into the medium with a resultant change in apparent Mr to 225,000, the size of the mature subunit found in plasma. The Mr 240,000 species was detected within 10 min of the start of labeling of cells, but factor VIIIR was not detected in the culture medium until approximately equal to 50 min. Treatment of the cells with tunicamycin resulted in a decrease in the apparent Mr of both species but did not grossly inhibit processing of precursor to product or the secretion of the latter. Thus, much or all N-linked glycosylation of factor VIIIR is not essential for these steps. Accumulation of factor VIIIR in the medium continued over a 24-hr period of cell labeling with [35S]-methionine, without significant net intracellular accumulation of the precursor, suggesting that a large storage pool of factor VIIIR is not present in endothelial cells under these conditions.