β protein C is not glycosylated at asparagine 329: The rate of translation may influence the frequency of usage at asparagine-x-cysteine sites

About 30% of human plasma protein C is smaller than the predominant form as judged by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. It has been suggested that this species, referred to as β protein C, is a degraded molecule. However, β protein C is secreted in culture by the HepG2 cell line and is present in plasma collected directly into numerous proteinase inhibitors; the percentage of β protein C does not change with time during culture or after blood collection. Neither thrombin, activated protein C, nor activated factor X converts the a form to β in the presence or absence of calcium and phospholipids. The NH₂-terminal sequences of the heavy chains of both forms are identical, and both release the same dodecapeptide and develop a functional active site when cleaved by thrombin. Both also react with antibodies to a synthetic COOH-terminal peptide. Timed digests with N-glycosidase are consistent with the interpretation that β protein C has three N-linked oligosaccharide chains whereas a protein C has four. It is asparagine 329 that is not glycosylated in β protein C since antibodies to a synthetic peptide based on the sequence around this amino acid react only with β protein C. This site is unique in having cysteine instead of serine or threonine 2 residues distal. It is likely that the sulfhydryl group can substitute for the usual hydroxyl group as a hydrogen bond acceptor for the glycosylation reaction only until it forms a disulfide bond. The percentage of protein C that is glycosylated at this site may therefore depend at least in part on the rate of disulfide bond formation which may in turn be related to the rate of protein synthesis.