Proteolytic processing of human protein C in swine mammary gland

Maturation of human Protein C (HPC) precursor to a zymogen in the liver requires endoproteolytic cleavages after a basic dipeptide, Lys-2-Arg-1 in the propeptide and Lys156-Arg157 connecting the light and heavy chain. Recombinant human Protein C (rHPC) was expressed in the mammary gland of transgenic swine and its proteolytic processing was monitored. We found that about 10-20% of rHPC purified from the milk still retained the propeptide and 30-40% was in the single-chain form, indicating inefficient proteolytic cleavage. This demonstrates that endoprotease(s) of the swine mammary epithelial cells do not process fully the precursor of heterologous protein. rHPC was fractionated by anion exchange chromatography and polypeptides with novel N-termini at positions -1,152 and 157 were detected in addition to the known N-termini at residues -24, +1, and 158. Since rHPC was found to be stable both in milk and after purification, it is possible that these new cleavages on the amino side of arginine at dipeptide sites Lys-2-Arg-1, Lys151-Arg152, and Lys156-Arg157 could have occurred in the mammary gland. Thus, our results suggest that a portion of HPC precursor was proteolytically processed in swine mammary gland differently than those in other expression systems.