Intracellular proteolytic processing of the heavy chain of rat pre-alpha-inhibitor - The COOH-terminal propeptide is required for coupling to bikunin

Pre-alpha-inhibitor is a serum protein consisting of two polypeptides named bikunin and heavy chain 3 (H3). Both polypeptides are synthesized in hepat ocytes and while passing through the Golgi complex, bikunin, which carries a chondroitin sulfate chain, becomes covalently linked to the COOH-terminal amino acid residue of H3 via its polysaccharide. Immediately prior to this reaction, a COOH-terminal propeptide of 33 kDa is cleaved off from the hea vy chain. Using COS-1 cells transfected with rat H3, we found that in the a bsence of bikunin, the cleaved propeptide remained bound to the heavy chain and that H3 lacking the propeptide sequence did not become linked to coexp ressed bikunin. Sequencing of H3 secreted from COS-1 cells showed that part of the molecules had a 12-amino acid residue long NH2-terminal propeptide. Cleavage of this propeptide, which occurred in the endoplasmic reticulum, was found to require basic amino acid residues at P-1, P-2, and P-6 suggest ing that it is mediated by a Golgi enzyme in transit. Deletion of the NH2-t erminal propeptide or blocking of its release affected neither transport no r coupling of the heavy chain to bikunin.